diff --git a/app/api/projects/[projectId]/board/comments/[commentId]/route.ts b/app/api/projects/[projectId]/board/comments/[commentId]/route.ts
new file mode 100644
index 0000000..b933155
--- /dev/null
+++ b/app/api/projects/[projectId]/board/comments/[commentId]/route.ts
@@ -0,0 +1,51 @@
+import { NextRequest, NextResponse } from 'next/server';
+import { getCurrentUser } from '@/lib/auth/getCurrentUser';
+import { createBoardService } from '@/lib/api/services';
+import { UnauthorizedError } from '@/lib/errors';
+import { handleApiError, jsonError } from '@/lib/api/handleError';
+
+export const runtime = 'nodejs';
+
+export async function PATCH(
+  request: NextRequest,
+  { params }: { params: Promise<{ projectId: string; commentId: string }> }
+) {
+  const user = await getCurrentUser();
+  if (!user) return handleApiError(new UnauthorizedError());
+  const { commentId } = await params;
+  let body: Record<string, unknown>;
+  try {
+    body = (await request.json()) as Record<string, unknown>;
+  } catch {
+    return jsonError(400, 'リクエスト本文が不正です');
+  }
+
+  const service = createBoardService();
+  try {
+    const comment = service.updateComment(
+      user.id,
+      Number(commentId),
+      String(body.bodyMd ?? '')
+    );
+    return NextResponse.json({ comment });
+  } catch (error) {
+    return handleApiError(error);
+  }
+}
+
+export async function DELETE(
+  _request: NextRequest,
+  { params }: { params: Promise<{ projectId: string; commentId: string }> }
+) {
+  const user = await getCurrentUser();
+  if (!user) return handleApiError(new UnauthorizedError());
+  const { commentId } = await params;
+
+  const service = createBoardService();
+  try {
+    service.deleteComment(user.id, Number(commentId));
+    return NextResponse.json({ ok: true });
+  } catch (error) {
+    return handleApiError(error);
+  }
+}
diff --git a/app/api/projects/[projectId]/board/threads/[threadId]/comments/route.ts b/app/api/projects/[projectId]/board/threads/[threadId]/comments/route.ts
new file mode 100644
index 0000000..0d4af9c
--- /dev/null
+++ b/app/api/projects/[projectId]/board/threads/[threadId]/comments/route.ts
@@ -0,0 +1,34 @@
+import { NextRequest, NextResponse } from 'next/server';
+import { getCurrentUser } from '@/lib/auth/getCurrentUser';
+import { createBoardService } from '@/lib/api/services';
+import { UnauthorizedError } from '@/lib/errors';
+import { handleApiError, jsonError } from '@/lib/api/handleError';
+
+export const runtime = 'nodejs';
+
+export async function POST(
+  request: NextRequest,
+  { params }: { params: Promise<{ projectId: string; threadId: string }> }
+) {
+  const user = await getCurrentUser();
+  if (!user) return handleApiError(new UnauthorizedError());
+  const { threadId } = await params;
+  let body: Record<string, unknown>;
+  try {
+    body = (await request.json()) as Record<string, unknown>;
+  } catch {
+    return jsonError(400, 'リクエスト本文が不正です');
+  }
+
+  const service = createBoardService();
+  try {
+    const comment = service.createComment(
+      user.id,
+      Number(threadId),
+      String(body.bodyMd ?? '')
+    );
+    return NextResponse.json({ comment }, { status: 201 });
+  } catch (error) {
+    return handleApiError(error);
+  }
+}
diff --git a/app/api/projects/[projectId]/board/threads/[threadId]/route.ts b/app/api/projects/[projectId]/board/threads/[threadId]/route.ts
new file mode 100644
index 0000000..88bdf2a
--- /dev/null
+++ b/app/api/projects/[projectId]/board/threads/[threadId]/route.ts
@@ -0,0 +1,75 @@
+import { NextRequest, NextResponse } from 'next/server';
+import { getCurrentUser } from '@/lib/auth/getCurrentUser';
+import { createBoardService } from '@/lib/api/services';
+import { UnauthorizedError } from '@/lib/errors';
+import { handleApiError, jsonError } from '@/lib/api/handleError';
+
+export const runtime = 'nodejs';
+
+export async function GET(
+  _request: NextRequest,
+  { params }: { params: Promise<{ projectId: string; threadId: string }> }
+) {
+  const user = await getCurrentUser();
+  if (!user) return handleApiError(new UnauthorizedError());
+  const { threadId } = await params;
+
+  const service = createBoardService();
+  try {
+    const thread = service.getThread(user.id, Number(threadId));
+    const comments = service.listComments(user.id, Number(threadId));
+    return NextResponse.json({ thread, comments });
+  } catch (error) {
+    return handleApiError(error);
+  }
+}
+
+export async function PATCH(
+  request: NextRequest,
+  { params }: { params: Promise<{ projectId: string; threadId: string }> }
+) {
+  const user = await getCurrentUser();
+  if (!user) return handleApiError(new UnauthorizedError());
+  const { threadId } = await params;
+  let body: Record<string, unknown>;
+  try {
+    body = (await request.json()) as Record<string, unknown>;
+  } catch {
+    return jsonError(400, 'リクエスト本文が不正です');
+  }
+
+  const service = createBoardService();
+  try {
+    const thread = service.updateThread(user.id, Number(threadId), {
+      title: typeof body.title === 'string' ? body.title : undefined,
+      bodyMd: typeof body.bodyMd === 'string' ? body.bodyMd : undefined,
+      category:
+        typeof body.category === 'string'
+          ? (body.category as never)
+          : undefined,
+      isPinned: typeof body.isPinned === 'number' ? body.isPinned : undefined,
+      isImportant:
+        typeof body.isImportant === 'number' ? body.isImportant : undefined,
+    });
+    return NextResponse.json({ thread });
+  } catch (error) {
+    return handleApiError(error);
+  }
+}
+
+export async function DELETE(
+  _request: NextRequest,
+  { params }: { params: Promise<{ projectId: string; threadId: string }> }
+) {
+  const user = await getCurrentUser();
+  if (!user) return handleApiError(new UnauthorizedError());
+  const { threadId } = await params;
+
+  const service = createBoardService();
+  try {
+    service.deleteThread(user.id, Number(threadId));
+    return NextResponse.json({ ok: true });
+  } catch (error) {
+    return handleApiError(error);
+  }
+}
diff --git a/app/api/projects/[projectId]/board/threads/route.ts b/app/api/projects/[projectId]/board/threads/route.ts
new file mode 100644
index 0000000..c9cbdb6
--- /dev/null
+++ b/app/api/projects/[projectId]/board/threads/route.ts
@@ -0,0 +1,57 @@
+import { NextRequest, NextResponse } from 'next/server';
+import { getCurrentUser } from '@/lib/auth/getCurrentUser';
+import { createBoardService } from '@/lib/api/services';
+import { UnauthorizedError } from '@/lib/errors';
+import { handleApiError, jsonError } from '@/lib/api/handleError';
+
+export const runtime = 'nodejs';
+
+export async function GET(
+  request: NextRequest,
+  { params }: { params: Promise<{ projectId: string }> }
+) {
+  const user = await getCurrentUser();
+  if (!user) return handleApiError(new UnauthorizedError());
+  const { projectId } = await params;
+  const page = Number(request.nextUrl.searchParams.get('page') ?? '1') || 1;
+  const search = request.nextUrl.searchParams.get('q') ?? undefined;
+
+  const service = createBoardService();
+  try {
+    return NextResponse.json(
+      service.listThreads(user.id, Number(projectId), { page, search })
+    );
+  } catch (error) {
+    return handleApiError(error);
+  }
+}
+
+export async function POST(
+  request: NextRequest,
+  { params }: { params: Promise<{ projectId: string }> }
+) {
+  const user = await getCurrentUser();
+  if (!user) return handleApiError(new UnauthorizedError());
+  const { projectId } = await params;
+  let body: Record<string, unknown>;
+  try {
+    body = (await request.json()) as Record<string, unknown>;
+  } catch {
+    return jsonError(400, 'リクエスト本文が不正です');
+  }
+
+  const service = createBoardService();
+  try {
+    const thread = service.createThread(user.id, Number(projectId), {
+      title: String(body.title ?? ''),
+      bodyMd: String(body.bodyMd ?? ''),
+      category:
+        typeof body.category === 'string'
+          ? (body.category as never)
+          : undefined,
+    });
+    return NextResponse.json({ thread }, { status: 201 });
+  } catch (error) {
+    return handleApiError(error);
+  }
+}
diff --git a/app/projects/[projectId]/board/[threadId]/page.tsx b/app/projects/[projectId]/board/[threadId]/page.tsx
new file mode 100644
index 0000000..49cdee2
--- /dev/null
+++ b/app/projects/[projectId]/board/[threadId]/page.tsx
@@ -0,0 +1,78 @@
+import { redirect } from 'next/navigation';
+import { getCurrentUser, toPublicUser } from '@/lib/auth/getCurrentUser';
+import { createBoardService } from '@/lib/api/services';
+import { Header } from '@/components/layout/Header';
+import { ProjectNav } from '@/components/layout/ProjectNav';
+import { MarkdownBody } from '@/components/board/MarkdownBody';
+import { CommentForm } from '@/components/board/CommentForm';
+import { ForbiddenError, NotFoundError } from '@/lib/errors';
+
+export const dynamic = 'force-dynamic';
+
+export default async function ThreadDetailPage({
+  params,
+}: {
+  params: Promise<{ projectId: string; threadId: string }>;
+}) {
+  const user = await getCurrentUser();
+  if (!user) redirect('/login');
+  const { projectId, threadId } = await params;
+
+  const boardService = createBoardService();
+  let thread;
+  let comments;
+  try {
+    thread = boardService.getThread(user.id, Number(threadId));
+    comments = boardService.listComments(user.id, Number(threadId));
+  } catch (error) {
+    if (error instanceof ForbiddenError || error instanceof NotFoundError) {
+      redirect(`/projects/${projectId}/board`);
+    }
+    throw error;
+  }
+
+  return (
+    <div className="min-h-screen bg-gray-50">
+      <Header user={toPublicUser(user)} />
+      <ProjectNav projectId={Number(projectId)} active="board" />
+      <main className="mx-auto max-w-3xl space-y-6 p-6">
+        <a
+          href={`/projects/${projectId}/board`}
+          className="text-sm text-blue-600 hover:underline"
+        >
+          ← 掲示板一覧へ
+        </a>
+        <div className="rounded-lg border bg-white p-6 shadow-sm">
+          <div className="flex items-center justify-between">
+            <h1 className="text-2xl font-bold">{thread.title}</h1>
+            {thread.category && (
+              <span className="rounded bg-gray-100 px-2 py-0.5 text-xs">
+                {thread.category}
+              </span>
+            )}
+          </div>
+          <div className="mt-4">
+            <MarkdownBody bodyMd={thread.bodyMd} />
+          </div>
+          <p className="mt-4 text-xs text-gray-400">
+            投稿: {thread.createdAt} / 更新: {thread.updatedAt}
+          </p>
+        </div>
+
+        <section className="space-y-4">
+          <h2 className="text-lg font-bold">コメント ({comments.total})</h2>
+          {comments.items.map((comment) => (
+            <div
+              key={comment.id}
+              className="rounded-lg border bg-white p-4 shadow-sm"
+            >
+              <MarkdownBody bodyMd={comment.bodyMd} />
+              <p className="mt-2 text-xs text-gray-400">{comment.createdAt}</p>
+            </div>
+          ))}
+          <CommentForm projectId={Number(projectId)} threadId={thread.id} />
+        </section>
+      </main>
+    </div>
+  );
+}
diff --git a/app/projects/[projectId]/board/page.tsx b/app/projects/[projectId]/board/page.tsx
new file mode 100644
index 0000000..4493d62
--- /dev/null
+++ b/app/projects/[projectId]/board/page.tsx
@@ -0,0 +1,77 @@
+import { redirect } from 'next/navigation';
+import { getCurrentUser, toPublicUser } from '@/lib/auth/getCurrentUser';
+import { createBoardService, createProjectService } from '@/lib/api/services';
+import { Header } from '@/components/layout/Header';
+import { ProjectNav } from '@/components/layout/ProjectNav';
+import { ThreadForm } from '@/components/board/ThreadForm';
+import { ForbiddenError, NotFoundError } from '@/lib/errors';
+
+export const dynamic = 'force-dynamic';
+
+export default async function BoardPage({
+  params,
+  searchParams,
+}: {
+  params: Promise<{ projectId: string }>;
+  searchParams: Promise<{ q?: string; page?: string }>;
+}) {
+  const user = await getCurrentUser();
+  if (!user) redirect('/login');
+  const { projectId } = await params;
+  const { q, page } = await searchParams;
+
+  const projectService = createProjectService();
+  let project: Awaited<ReturnType<typeof projectService.getProject>>;
+  try {
+    project = projectService.getProject(user.id, Number(projectId));
+  } catch (error) {
+    if (error instanceof ForbiddenError || error instanceof NotFoundError) {
+      redirect('/dashboard');
+    }
+    throw error;
+  }
+
+  const boardService = createBoardService();
+  const { items } = boardService.listThreads(user.id, project.id, {
+    page: Number(page ?? '1') || 1,
+    search: q,
+  });
+
+  return (
+    <div className="min-h-screen bg-gray-50">
+      <Header user={toPublicUser(user)} />
+      <ProjectNav projectId={project.id} active="board" />
+      <main className="mx-auto max-w-3xl space-y-6 p-6">
+        <h1 className="text-2xl font-bold">掲示板</h1>
+        <ThreadForm projectId={project.id} />
+        <section className="space-y-3">
+          {items.length === 0 ? (
+            <p className="text-sm text-gray-400">スレッドはありません。</p>
+          ) : (
+            items.map((thread) => (
+              <a
+                key={thread.id}
+                href={`/projects/${project.id}/board/${thread.id}`}
+                className="block rounded-lg border bg-white p-4 shadow-sm hover:shadow-md"
+              >
+                <div className="flex items-center justify-between">
+                  <h3 className="font-semibold text-gray-800">
+                    {thread.isPinned === 1 && '📌 '}
+                    {thread.isImportant === 1 && '❗ '}
+                    {thread.title}
+                  </h3>
+                  {thread.category && (
+                    <span className="rounded bg-gray-100 px-2 py-0.5 text-xs">
+                      {thread.category}
+                    </span>
+                  )}
+                </div>
+                <p className="mt-1 text-xs text-gray-400">{thread.createdAt}</p>
+              </a>
+            ))
+          )}
+        </section>
+      </main>
+    </div>
+  );
+}
diff --git a/components/board/CommentForm.tsx b/components/board/CommentForm.tsx
new file mode 100644
index 0000000..c184282
--- /dev/null
+++ b/components/board/CommentForm.tsx
@@ -0,0 +1,65 @@
+'use client';
+
+import { useState, type FormEvent } from 'react';
+import { useRouter } from 'next/navigation';
+
+export function CommentForm({
+  projectId,
+  threadId,
+}: {
+  projectId: number;
+  threadId: number;
+}) {
+  const router = useRouter();
+  const [bodyMd, setBodyMd] = useState('');
+  const [loading, setLoading] = useState(false);
+  const [error, setError] = useState<string | null>(null);
+
+  async function onSubmit(event: FormEvent<HTMLFormElement>) {
+    event.preventDefault();
+    setLoading(true);
+    setError(null);
+    const res = await fetch(
+      `/api/projects/${projectId}/board/threads/${threadId}/comments`,
+      {
+        method: 'POST',
+        headers: { 'Content-Type': 'application/json' },
+        body: JSON.stringify({ bodyMd }),
+      }
+    );
+    setLoading(false);
+    if (res.ok) {
+      setBodyMd('');
+      router.refresh();
+    } else {
+      const b = (await res.json().catch(() => null)) as {
+        error?: { message?: string };
+      } | null;
+      setError(b?.error?.message ?? '投稿に失敗しました');
+    }
+  }
+
+  return (
+    <form onSubmit={onSubmit} className="space-y-2">
+      <textarea
+        placeholder="コメント（Markdown）"
+        value={bodyMd}
+        onChange={(e) => setBodyMd(e.target.value)}
+        className="min-h-[80px] w-full rounded border px-3 py-2"
+        required
+      />
+      {error && (
+        <p className="text-sm text-red-600" role="alert">
+          {error}
+        </p>
+      )}
+      <button
+        type="submit"
+        disabled={loading}
+        className="rounded bg-blue-600 px-4 py-2 text-sm font-medium text-white hover:bg-blue-700 disabled:opacity-50"
+      >
+        {loading ? '投稿中...' : 'コメント投稿'}
+      </button>
+    </form>
+  );
+}
diff --git a/components/board/MarkdownBody.tsx b/components/board/MarkdownBody.tsx
new file mode 100644
index 0000000..651e7fa
--- /dev/null
+++ b/components/board/MarkdownBody.tsx
@@ -0,0 +1,20 @@
+import ReactMarkdown from 'react-markdown';
+import remarkGfm from 'remark-gfm';
+import rehypeSanitize from 'rehype-sanitize';
+
+/**
+ * Markdown本文を安全にレンダリングする。
+ * HTML直接入力は無効化し、rehype-sanitize でサニタイズする。
+ */
+export function MarkdownBody({ bodyMd }: { bodyMd: string }) {
+  return (
+    <div className="prose prose-sm max-w-none">
+      <ReactMarkdown
+        remarkPlugins={[remarkGfm]}
+        rehypePlugins={[rehypeSanitize]}
+      >
+        {bodyMd}
+      </ReactMarkdown>
+    </div>
+  );
+}
diff --git a/components/board/ThreadForm.tsx b/components/board/ThreadForm.tsx
new file mode 100644
index 0000000..064d3c0
--- /dev/null
+++ b/components/board/ThreadForm.tsx
@@ -0,0 +1,99 @@
+'use client';
+
+import { useState, type FormEvent } from 'react';
+import { useRouter } from 'next/navigation';
+
+const CATEGORIES = [
+  { value: '', label: 'なし' },
+  { value: 'notice', label: 'notice' },
+  { value: 'spec', label: 'spec' },
+  { value: 'minutes', label: 'minutes' },
+  { value: 'question', label: 'question' },
+  { value: 'decision', label: 'decision' },
+  { value: 'trouble', label: 'trouble' },
+  { value: 'memo', label: 'memo' },
+] as const;
+
+export function ThreadForm({ projectId }: { projectId: number }) {
+  const router = useRouter();
+  const [title, setTitle] = useState('');
+  const [bodyMd, setBodyMd] = useState('');
+  const [category, setCategory] = useState('');
+  const [error, setError] = useState<string | null>(null);
+  const [loading, setLoading] = useState(false);
+
+  async function onSubmit(event: FormEvent<HTMLFormElement>) {
+    event.preventDefault();
+    setLoading(true);
+    setError(null);
+    const res = await fetch(`/api/projects/${projectId}/board/threads`, {
+      method: 'POST',
+      headers: { 'Content-Type': 'application/json' },
+      body: JSON.stringify({
+        title,
+        bodyMd,
+        category: category || undefined,
+      }),
+    });
+    setLoading(false);
+    if (res.ok) {
+      const data = (await res.json()) as { thread: { id: number } };
+      router.push(`/projects/${projectId}/board/${data.thread.id}`);
+      router.refresh();
+    } else {
+      const b = (await res.json().catch(() => null)) as {
+        error?: { message?: string };
+      } | null;
+      setError(b?.error?.message ?? '作成に失敗しました');
+    }
+  }
+
+  return (
+    <form
+      onSubmit={onSubmit}
+      className="space-y-3 rounded-lg border bg-white p-4 shadow-sm"
+    >
+      <div className="flex gap-2">
+        <input
+          type="text"
+          placeholder="タイトル"
+          value={title}
+          onChange={(e) => setTitle(e.target.value)}
+          className="flex-1 rounded border px-3 py-2"
+          required
+          maxLength={200}
+        />
+        <select
+          value={category}
+          onChange={(e) => setCategory(e.target.value)}
+          className="rounded border px-2 py-2"
+        >
+          {CATEGORIES.map((c) => (
+            <option key={c.value} value={c.value}>
+              {c.label}
+            </option>
+          ))}
+        </select>
+      </div>
+      <textarea
+        placeholder="本文（Markdown）"
+        value={bodyMd}
+        onChange={(e) => setBodyMd(e.target.value)}
+        className="min-h-[120px] w-full rounded border px-3 py-2"
+        required
+      />
+      {error && (
+        <p className="text-sm text-red-600" role="alert">
+          {error}
+        </p>
+      )}
+      <button
+        type="submit"
+        disabled={loading}
+        className="rounded bg-blue-600 px-4 py-2 font-medium text-white hover:bg-blue-700 disabled:opacity-50"
+      >
+        {loading ? '作成中...' : 'スレッド作成'}
+      </button>
+    </form>
+  );
+}
diff --git a/components/layout/ProjectNav.tsx b/components/layout/ProjectNav.tsx
index 396c252..ca65d1a 100644
--- a/components/layout/ProjectNav.tsx
+++ b/components/layout/ProjectNav.tsx
@@ -1,5 +1,6 @@
 const NAV_ITEMS = [
   { href: '', label: '概要' },
+  { href: '/board', label: '掲示板' },
   { href: '/members', label: 'メンバー' },
   { href: '/activity', label: 'アクティビティ' },
   { href: '/settings', label: '設定' },
@@ -14,10 +15,11 @@ export function ProjectNav({
   active,
 }: {
   projectId: number;
-  active: 'overview' | 'members' | 'activity' | 'settings';
+  active: 'overview' | 'board' | 'members' | 'activity' | 'settings';
 }) {
   const activeMap: Record<string, boolean> = {
     overview: active === 'overview',
+    board: active === 'board',
     members: active === 'members',
     activity: active === 'activity',
     settings: active === 'settings',
diff --git a/lib/api/services.ts b/lib/api/services.ts
index c86ff88..1e3cb39 100644
--- a/lib/api/services.ts
+++ b/lib/api/services.ts
@@ -7,6 +7,8 @@ import { UserRepository } from '@/repositories/UserRepository';
 import { ProjectService } from '@/services/ProjectService';
 import { NotificationService } from '@/services/NotificationService';
 import { ActivityLogService } from '@/services/ActivityLogService';
+import { BoardService } from '@/services/BoardService';
+import { BoardRepository } from '@/repositories/BoardRepository';
 
 /**
  * Route Handler用に各Repository/Serviceを構築するファクトリ。
@@ -30,6 +32,16 @@ export function createActivityLogService(): ActivityLogService {
   return new ActivityLogService(new ActivityLogRepository(getDb()));
 }
 
+export function createBoardService(): BoardService {
+  const db = getDb();
+  return new BoardService(
+    new BoardRepository(db),
+    new ProjectMemberRepository(db),
+    new NotificationService(new NotificationRepository(db)),
+    new ActivityLogService(new ActivityLogRepository(db))
+  );
+}
+
 export function createUserRepository(): UserRepository {
   return new UserRepository(getDb());
 }
diff --git a/repositories/BoardRepository.ts b/repositories/BoardRepository.ts
new file mode 100644
index 0000000..4595f16
--- /dev/null
+++ b/repositories/BoardRepository.ts
@@ -0,0 +1,265 @@
+import type { SqliteDatabase } from '@/lib/db/sqlite';
+import type { BoardCategory, BoardComment, BoardThread } from '@/lib/types';
+
+const DEFAULT_PAGE_SIZE = 20;
+
+export interface Paginated<T> {
+  items: T[];
+  total: number;
+}
+
+interface BoardThreadRow {
+  id: number;
+  project_id: number;
+  title: string;
+  body_md: string;
+  author_id: number;
+  category: string | null;
+  is_pinned: number;
+  is_important: number;
+  created_at: string;
+  updated_at: string;
+  deleted_at: string | null;
+}
+
+interface BoardCommentRow {
+  id: number;
+  thread_id: number;
+  author_id: number;
+  body_md: string;
+  created_at: string;
+  updated_at: string;
+  deleted_at: string | null;
+}
+
+function mapThread(row: BoardThreadRow): BoardThread {
+  return {
+    id: row.id,
+    projectId: row.project_id,
+    title: row.title,
+    bodyMd: row.body_md,
+    authorId: row.author_id,
+    category: row.category as BoardCategory | null,
+    isPinned: row.is_pinned,
+    isImportant: row.is_important,
+    createdAt: row.created_at,
+    updatedAt: row.updated_at,
+    deletedAt: row.deleted_at,
+  };
+}
+
+function mapComment(row: BoardCommentRow): BoardComment {
+  return {
+    id: row.id,
+    threadId: row.thread_id,
+    authorId: row.author_id,
+    bodyMd: row.body_md,
+    createdAt: row.created_at,
+    updatedAt: row.updated_at,
+    deletedAt: row.deleted_at,
+  };
+}
+
+export interface CreateThreadInput {
+  projectId: number;
+  title: string;
+  bodyMd: string;
+  authorId: number;
+  category: BoardCategory | null;
+}
+
+export interface UpdateThreadInput {
+  title?: string;
+  bodyMd?: string;
+  category?: BoardCategory | null;
+  isPinned?: number;
+  isImportant?: number;
+}
+
+export interface ListThreadsOptions {
+  page?: number;
+  pageSize?: number;
+  search?: string;
+}
+
+export class BoardRepository {
+  constructor(private readonly db: SqliteDatabase) {}
+
+  findThreads(
+    projectId: number,
+    opts: ListThreadsOptions = {}
+  ): Paginated<BoardThread> {
+    const page = opts.page ?? 1;
+    const pageSize = opts.pageSize ?? DEFAULT_PAGE_SIZE;
+    const offset = (page - 1) * pageSize;
+    const search = opts.search?.trim();
+
+    if (search) {
+      const like = `%${search}%`;
+      const items = this.db.query<BoardThreadRow>(
+        `SELECT * FROM board_threads
+         WHERE project_id = @projectId AND deleted_at IS NULL
+           AND (title LIKE @like OR body_md LIKE @like)
+         ORDER BY is_pinned DESC, created_at DESC, id DESC
+         LIMIT @pageSize OFFSET @offset`,
+        { projectId, like, pageSize, offset }
+      );
+      const total = this.db.get<{ count: number }>(
+        `SELECT COUNT(*) AS count FROM board_threads
+         WHERE project_id = @projectId AND deleted_at IS NULL
+           AND (title LIKE @like OR body_md LIKE @like)`,
+        { projectId, like }
+      );
+      return { items: items.map(mapThread), total: total?.count ?? 0 };
+    }
+
+    const items = this.db.query<BoardThreadRow>(
+      `SELECT * FROM board_threads
+       WHERE project_id = @projectId AND deleted_at IS NULL
+       ORDER BY is_pinned DESC, created_at DESC, id DESC
+       LIMIT @pageSize OFFSET @offset`,
+      { projectId, pageSize, offset }
+    );
+    const total = this.db.get<{ count: number }>(
+      'SELECT COUNT(*) AS count FROM board_threads WHERE project_id = @projectId AND deleted_at IS NULL',
+      { projectId }
+    );
+    return { items: items.map(mapThread), total: total?.count ?? 0 };
+  }
+
+  findThreadById(id: number): BoardThread | null {
+    const row = this.db.get<BoardThreadRow>(
+      'SELECT * FROM board_threads WHERE id = @id AND deleted_at IS NULL',
+      { id }
+    );
+    return row ? mapThread(row) : null;
+  }
+
+  createThread(input: CreateThreadInput): BoardThread {
+    const now = new Date().toISOString();
+    const result = this.db.execute(
+      `INSERT INTO board_threads (project_id, title, body_md, author_id, category, is_pinned, is_important, created_at, updated_at, deleted_at)
+       VALUES (@projectId, @title, @bodyMd, @authorId, @category, 0, 0, @createdAt, @updatedAt, NULL)`,
+      {
+        projectId: input.projectId,
+        title: input.title,
+        bodyMd: input.bodyMd,
+        authorId: input.authorId,
+        category: input.category,
+        createdAt: now,
+        updatedAt: now,
+      }
+    );
+    const created = this.findThreadById(Number(result.lastInsertRowid));
+    if (!created) throw new Error('Failed to create thread');
+    return created;
+  }
+
+  updateThread(id: number, input: UpdateThreadInput): BoardThread | null {
+    const fields: string[] = ['updated_at = @updatedAt'];
+    const params: Record<string, unknown> = {
+      updatedAt: new Date().toISOString(),
+      id,
+    };
+    if (input.title !== undefined) {
+      fields.push('title = @title');
+      params.title = input.title;
+    }
+    if (input.bodyMd !== undefined) {
+      fields.push('body_md = @bodyMd');
+      params.bodyMd = input.bodyMd;
+    }
+    if (input.category !== undefined) {
+      fields.push('category = @category');
+      params.category = input.category;
+    }
+    if (input.isPinned !== undefined) {
+      fields.push('is_pinned = @isPinned');
+      params.isPinned = input.isPinned;
+    }
+    if (input.isImportant !== undefined) {
+      fields.push('is_important = @isImportant');
+      params.isImportant = input.isImportant;
+    }
+    this.db.execute(
+      `UPDATE board_threads SET ${fields.join(', ')} WHERE id = @id AND deleted_at IS NULL`,
+      params
+    );
+    return this.findThreadById(id);
+  }
+
+  deleteThread(id: number): boolean {
+    const result = this.db.execute(
+      'UPDATE board_threads SET deleted_at = @now WHERE id = @id AND deleted_at IS NULL',
+      { now: new Date().toISOString(), id }
+    );
+    return result.changes > 0;
+  }
+
+  findCommentsByThread(
+    threadId: number,
+    page: number = 1,
+    pageSize: number = DEFAULT_PAGE_SIZE
+  ): Paginated<BoardComment> {
+    const offset = (page - 1) * pageSize;
+    const items = this.db.query<BoardCommentRow>(
+      `SELECT * FROM board_comments
+       WHERE thread_id = @threadId AND deleted_at IS NULL
+       ORDER BY created_at ASC, id ASC
+       LIMIT @pageSize OFFSET @offset`,
+      { threadId, pageSize, offset }
+    );
+    const total = this.db.get<{ count: number }>(
+      'SELECT COUNT(*) AS count FROM board_comments WHERE thread_id = @threadId AND deleted_at IS NULL',
+      { threadId }
+    );
+    return { items: items.map(mapComment), total: total?.count ?? 0 };
+  }
+
+  findCommentById(id: number): BoardComment | null {
+    const row = this.db.get<BoardCommentRow>(
+      'SELECT * FROM board_comments WHERE id = @id AND deleted_at IS NULL',
+      { id }
+    );
+    return row ? mapComment(row) : null;
+  }
+
+  createComment(input: {
+    threadId: number;
+    authorId: number;
+    bodyMd: string;
+  }): BoardComment {
+    const now = new Date().toISOString();
+    const result = this.db.execute(
+      `INSERT INTO board_comments (thread_id, author_id, body_md, created_at, updated_at, deleted_at)
+       VALUES (@threadId, @authorId, @bodyMd, @createdAt, @updatedAt, NULL)`,
+      {
+        threadId: input.threadId,
+        authorId: input.authorId,
+        bodyMd: input.bodyMd,
+        createdAt: now,
+        updatedAt: now,
+      }
+    );
+    const created = this.findCommentById(Number(result.lastInsertRowid));
+    if (!created) throw new Error('Failed to create comment');
+    return created;
+  }
+
+  updateComment(id: number, bodyMd: string): BoardComment | null {
+    this.db.execute(
+      `UPDATE board_comments SET body_md = @bodyMd, updated_at = @updatedAt
+       WHERE id = @id AND deleted_at IS NULL`,
+      { bodyMd, updatedAt: new Date().toISOString(), id }
+    );
+    return this.findCommentById(id);
+  }
+
+  deleteComment(id: number): boolean {
+    const result = this.db.execute(
+      'UPDATE board_comments SET deleted_at = @now WHERE id = @id AND deleted_at IS NULL',
+      { now: new Date().toISOString(), id }
+    );
+    return result.changes > 0;
+  }
+}
diff --git a/services/BoardService.ts b/services/BoardService.ts
new file mode 100644
index 0000000..f73b11c
--- /dev/null
+++ b/services/BoardService.ts
@@ -0,0 +1,235 @@
+import {
+  BoardRepository,
+  type ListThreadsOptions,
+} from '@/repositories/BoardRepository';
+import type { Paginated } from '@/repositories/BoardRepository';
+import { ProjectMemberRepository } from '@/repositories/ProjectMemberRepository';
+import { NotificationService } from '@/services/NotificationService';
+import { ActivityLogService } from '@/services/ActivityLogService';
+import { ForbiddenError, NotFoundError, ValidationError } from '@/lib/errors';
+import type { BoardCategory, BoardComment, BoardThread } from '@/lib/types';
+
+const VALID_CATEGORIES: BoardCategory[] = [
+  'notice',
+  'spec',
+  'minutes',
+  'question',
+  'decision',
+  'trouble',
+  'memo',
+];
+
+export interface CreateThreadInput {
+  title: string;
+  bodyMd: string;
+  category?: BoardCategory;
+}
+
+export interface UpdateThreadInput {
+  title?: string;
+  bodyMd?: string;
+  category?: BoardCategory;
+  isPinned?: number;
+  isImportant?: number;
+}
+
+/**
+ * 掲示板の業務ロジックを担うService。
+ * 権限チェック(プロジェクト参加者のみ)、スレッド/コメントCRUD、
+ * コメント追加時の通知(board_commented)とアクティビティログ記録を行う。
+ */
+export class BoardService {
+  constructor(
+    private readonly boardRepository: BoardRepository,
+    private readonly projectMemberRepository: ProjectMemberRepository,
+    private readonly notificationService: NotificationService,
+    private readonly activityLogService: ActivityLogService
+  ) {}
+
+  listThreads(
+    actorId: number,
+    projectId: number,
+    opts: ListThreadsOptions = {}
+  ): Paginated<BoardThread> {
+    this.requireMember(projectId, actorId);
+    return this.boardRepository.findThreads(projectId, opts);
+  }
+
+  getThread(actorId: number, threadId: number): BoardThread {
+    const thread = this.boardRepository.findThreadById(threadId);
+    if (!thread) throw new NotFoundError('BoardThread', threadId);
+    this.requireMember(thread.projectId, actorId);
+    return thread;
+  }
+
+  createThread(
+    actorId: number,
+    projectId: number,
+    input: CreateThreadInput
+  ): BoardThread {
+    this.requireMember(projectId, actorId);
+    this.validateThreadInput(input.title, input.bodyMd, input.category);
+    const thread = this.boardRepository.createThread({
+      projectId,
+      title: input.title,
+      bodyMd: input.bodyMd,
+      authorId: actorId,
+      category: input.category ?? null,
+    });
+    this.activityLogService.logActivity({
+      projectId,
+      actorId,
+      action: 'board_posted',
+      targetType: 'thread',
+      targetId: thread.id,
+    });
+    return thread;
+  }
+
+  updateThread(
+    actorId: number,
+    threadId: number,
+    input: UpdateThreadInput
+  ): BoardThread {
+    const thread = this.getThread(actorId, threadId);
+    this.requireAuthorOrAdmin(thread.projectId, actorId, thread.authorId);
+    if (
+      input.title !== undefined ||
+      input.bodyMd !== undefined ||
+      input.category !== undefined
+    ) {
+      this.validateThreadInput(
+        input.title ?? thread.title,
+        input.bodyMd ?? thread.bodyMd,
+        input.category
+      );
+    }
+    const updated = this.boardRepository.updateThread(threadId, {
+      title: input.title,
+      bodyMd: input.bodyMd,
+      category: input.category,
+      isPinned: input.isPinned,
+      isImportant: input.isImportant,
+    });
+    if (!updated) throw new NotFoundError('BoardThread', threadId);
+    return updated;
+  }
+
+  deleteThread(actorId: number, threadId: number): void {
+    const thread = this.getThread(actorId, threadId);
+    this.requireAuthorOrAdmin(thread.projectId, actorId, thread.authorId);
+    this.boardRepository.deleteThread(threadId);
+  }
+
+  listComments(
+    actorId: number,
+    threadId: number,
+    page: number = 1
+  ): Paginated<BoardComment> {
+    const thread = this.getThread(actorId, threadId);
+    return this.boardRepository.findCommentsByThread(thread.id, page);
+  }
+
+  createComment(
+    actorId: number,
+    threadId: number,
+    bodyMd: string
+  ): BoardComment {
+    const thread = this.getThread(actorId, threadId);
+    if (!bodyMd.trim()) {
+      throw new ValidationError('コメント本文を入力してください', 'bodyMd');
+    }
+    const comment = this.boardRepository.createComment({
+      threadId: thread.id,
+      authorId: actorId,
+      bodyMd,
+    });
+    // スレッド投稿者へ通知(自分自身へのコメントは通知しない)
+    if (thread.authorId !== actorId) {
+      this.notificationService.notifyOnEvent({
+        type: 'board_commented',
+        projectId: thread.projectId,
+        title: `スレッド「${thread.title}」にコメントがつきました`,
+        body: bodyMd.slice(0, 100),
+        threadAuthorId: thread.authorId,
+      });
+    }
+    this.activityLogService.logActivity({
+      projectId: thread.projectId,
+      actorId,
+      action: 'comment_added',
+      targetType: 'comment',
+      targetId: comment.id,
+    });
+    return comment;
+  }
+
+  updateComment(
+    actorId: number,
+    commentId: number,
+    bodyMd: string
+  ): BoardComment {
+    const comment = this.boardRepository.findCommentById(commentId);
+    if (!comment) throw new NotFoundError('BoardComment', commentId);
+    this.getThread(actorId, comment.threadId);
+    if (comment.authorId !== actorId) {
+      throw new ForbiddenError('自分のコメントのみ編集できます');
+    }
+    if (!bodyMd.trim()) {
+      throw new ValidationError('コメント本文を入力してください', 'bodyMd');
+    }
+    const updated = this.boardRepository.updateComment(commentId, bodyMd);
+    if (!updated) throw new NotFoundError('BoardComment', commentId);
+    return updated;
+  }
+
+  deleteComment(actorId: number, commentId: number): void {
+    const comment = this.boardRepository.findCommentById(commentId);
+    if (!comment) throw new NotFoundError('BoardComment', commentId);
+    const thread = this.getThread(actorId, comment.threadId);
+    this.requireAuthorOrAdmin(thread.projectId, actorId, comment.authorId);
+    this.boardRepository.deleteComment(commentId);
+  }
+
+  private requireMember(projectId: number, actorId: number): void {
+    if (!this.projectMemberRepository.isMember(projectId, actorId)) {
+      throw new ForbiddenError('プロジェクトに参加していません');
+    }
+  }
+
+  private requireAuthorOrAdmin(
+    projectId: number,
+    actorId: number,
+    authorId: number
+  ): void {
+    if (actorId === authorId) return;
+    const role = this.projectMemberRepository.getRole(projectId, actorId);
+    if (role !== 'admin') {
+      throw new ForbiddenError(
+        '投稿者またはプロジェクト管理者のみ操作できます'
+      );
+    }
+  }
+
+  private validateThreadInput(
+    title: string,
+    bodyMd: string,
+    category?: BoardCategory
+  ): void {
+    if (!title.trim()) {
+      throw new ValidationError('タイトルを入力してください', 'title');
+    }
+    if (title.length > 200) {
+      throw new ValidationError(
+        'タイトルは200文字以内で入力してください',
+        'title'
+      );
+    }
+    if (!bodyMd.trim()) {
+      throw new ValidationError('本文を入力してください', 'bodyMd');
+    }
+    if (category && !VALID_CATEGORIES.includes(category)) {
+      throw new ValidationError('無効なカテゴリです', 'category');
+    }
+  }
+}
diff --git a/tests/e2e/board.spec.ts b/tests/e2e/board.spec.ts
new file mode 100644
index 0000000..a01f3b1
--- /dev/null
+++ b/tests/e2e/board.spec.ts
@@ -0,0 +1,80 @@
+import { test, expect } from '@playwright/test';
+
+function unique(prefix: string): string {
+  return `${prefix}-${Date.now()}-${Math.floor(Math.random() * 1_000_000)}`;
+}
+
+async function setupOwner(page: import('@playwright/test').Page) {
+  const email = unique('owner') + '@example.com';
+  await page.goto('/login');
+  await page.getByRole('button', { name: '新規登録はこちら' }).click();
+  await page.getByLabel('表示名').fill('Owner');
+  await page.getByLabel('メールアドレス').fill(email);
+  await page.getByLabel('パスワード').fill('password123');
+  await page.getByRole('button', { name: '登録する' }).click();
+  await expect(page).toHaveURL(/\/dashboard/);
+
+  await page.getByLabel('プロジェクト名').fill(unique('Proj'));
+  await page.getByRole('button', { name: '新規プロジェクト' }).click();
+  await expect(page).toHaveURL(/\/projects\/\d+$/);
+  return Number(page.url().match(/\/projects\/(\d+)/)![1]);
+}
+
+test.describe('board', () => {
+  test('create thread, view detail, comment, and search', async ({ page }) => {
+    const projectId = await setupOwner(page);
+    const title = unique('Thread');
+
+    // スレッド作成(API)
+    const res = await page.request.post(
+      `/api/projects/${projectId}/board/threads`,
+      {
+        data: {
+          title,
+          bodyMd: '# Hello\n\nthis is **markdown**',
+          category: 'notice',
+        },
+      }
+    );
+    expect(res.ok()).toBeTruthy();
+    const { thread } = (await res.json()) as { thread: { id: number } };
+
+    // 一覧に表示される
+    await page.goto(`/projects/${projectId}/board`);
+    await expect(
+      page.getByRole('link', { name: new RegExp(title) })
+    ).toBeVisible();
+
+    // 詳細: Markdown本文が表示される
+    await page.goto(`/projects/${projectId}/board/${thread.id}`);
+    await expect(
+      page.getByRole('heading', { name: 'Hello', level: 1 })
+    ).toBeVisible();
+    await expect(page.getByText('markdown')).toBeVisible();
+
+    // コメント追加(API) → 詳細に表示される
+    const commentRes = await page.request.post(
+      `/api/projects/${projectId}/board/threads/${thread.id}/comments`,
+      { data: { bodyMd: 'first comment' } }
+    );
+    expect(commentRes.ok()).toBeTruthy();
+    await page.reload();
+    await expect(page.getByText('first comment')).toBeVisible();
+
+    // 検索: 別スレッドを作成し、q で絞り込める
+    const otherTitle = unique('ZZZ');
+    await page.request.post(`/api/projects/${projectId}/board/threads`, {
+      data: { title: otherTitle, bodyMd: 'body', category: 'question' },
+    });
+    await page.goto(
+      `/projects/${projectId}/board?q=${encodeURIComponent(otherTitle)}`
+    );
+    await expect(
+      page.getByRole('link', { name: new RegExp(otherTitle) })
+    ).toBeVisible();
+    // 前のスレッドは検索結果に含まれない
+    await expect(
+      page.getByRole('link', { name: new RegExp(title) })
+    ).toHaveCount(0);
+  });
+});
diff --git a/tests/unit/repositories/BoardRepository.test.ts b/tests/unit/repositories/BoardRepository.test.ts
new file mode 100644
index 0000000..81b7efc
--- /dev/null
+++ b/tests/unit/repositories/BoardRepository.test.ts
@@ -0,0 +1,127 @@
+import { describe, it, expect, beforeEach, afterEach } from 'vitest';
+import { createMigratedTestDb } from '@/tests/helpers/db';
+import type { SqliteDatabase } from '@/lib/db/sqlite';
+import { UserRepository } from '@/repositories/UserRepository';
+import { ProjectRepository } from '@/repositories/ProjectRepository';
+import { ProjectMemberRepository } from '@/repositories/ProjectMemberRepository';
+import { BoardRepository } from '@/repositories/BoardRepository';
+
+describe('BoardRepository', () => {
+  let db: SqliteDatabase;
+  let repo: BoardRepository;
+  let projectId: number;
+  let authorId: number;
+
+  beforeEach(() => {
+    db = createMigratedTestDb();
+    repo = new BoardRepository(db);
+    authorId = new UserRepository(db).create({
+      name: 'A',
+      email: 'a@example.com',
+      passwordHash: 'h',
+    }).id;
+    projectId = new ProjectRepository(db).create({
+      name: 'P',
+      ownerId: authorId,
+    }).id;
+    new ProjectMemberRepository(db).add(projectId, authorId, 'admin');
+  });
+
+  afterEach(() => db.close());
+
+  function createThread(title: string, body = 'body') {
+    return repo.createThread({
+      projectId,
+      title,
+      bodyMd: body,
+      authorId,
+      category: 'notice',
+    });
+  }
+
+  it('creates and finds a thread', () => {
+    const t = createThread('T1');
+    expect(repo.findThreadById(t.id)?.title).toBe('T1');
+  });
+
+  it('lists threads pinned-first then newest', () => {
+    const t1 = createThread('old');
+    const t2 = createThread('new');
+    repo.updateThread(t1.id, { isPinned: 1 });
+
+    const { items } = repo.findThreads(projectId);
+    expect(items[0].id).toBe(t1.id);
+    expect(items[1].id).toBe(t2.id);
+  });
+
+  it('excludes soft-deleted threads', () => {
+    const t = createThread('T');
+    repo.deleteThread(t.id);
+    expect(repo.findThreadById(t.id)).toBeNull();
+    expect(repo.findThreads(projectId).total).toBe(0);
+  });
+
+  it('isolates threads by project', () => {
+    createThread('mine');
+    const otherProject = new ProjectRepository(db).create({
+      name: 'P2',
+      ownerId: authorId,
+    }).id;
+    repo.createThread({
+      projectId: otherProject,
+      title: 'theirs',
+      bodyMd: 'b',
+      authorId,
+      category: null,
+    });
+
+    expect(repo.findThreads(projectId).total).toBe(1);
+    expect(repo.findThreads(otherProject).total).toBe(1);
+  });
+
+  it('searches threads by title and body', () => {
+    createThread('Important notice', 'body');
+    createThread('Other', 'special keyword');
+
+    expect(repo.findThreads(projectId, { search: 'important' }).total).toBe(1);
+    expect(repo.findThreads(projectId, { search: 'keyword' }).total).toBe(1);
+    expect(repo.findThreads(projectId, { search: 'nomatch' }).total).toBe(0);
+  });
+
+  it('paginates threads', () => {
+    for (let i = 0; i < 5; i++) createThread(`t${i}`);
+    expect(
+      repo.findThreads(projectId, { page: 1, pageSize: 2 }).items
+    ).toHaveLength(2);
+    expect(repo.findThreads(projectId, { page: 1, pageSize: 2 }).total).toBe(5);
+  });
+
+  it('creates and lists comments (oldest first)', () => {
+    const t = createThread('T');
+    const c1 = repo.createComment({
+      threadId: t.id,
+      authorId,
+      bodyMd: 'first',
+    });
+    repo.createComment({ threadId: t.id, authorId, bodyMd: 'second' });
+
+    const { items, total } = repo.findCommentsByThread(t.id);
+    expect(total).toBe(2);
+    expect(items[0].id).toBe(c1.id);
+  });
+
+  it('excludes soft-deleted comments', () => {
+    const t = createThread('T');
+    const c = repo.createComment({ threadId: t.id, authorId, bodyMd: 'x' });
+    repo.deleteComment(c.id);
+    expect(repo.findCommentById(c.id)).toBeNull();
+    expect(repo.findCommentsByThread(t.id).total).toBe(0);
+  });
+
+  it('updates a comment body', () => {
+    const t = createThread('T');
+    const c = repo.createComment({ threadId: t.id, authorId, bodyMd: 'x' });
+    const updated = repo.updateComment(c.id, 'updated');
+    expect(updated?.bodyMd).toBe('updated');
+  });
+});
diff --git a/tests/unit/services/BoardService.test.ts b/tests/unit/services/BoardService.test.ts
new file mode 100644
index 0000000..a531497
--- /dev/null
+++ b/tests/unit/services/BoardService.test.ts
@@ -0,0 +1,170 @@
+import { describe, it, expect, beforeEach, afterEach } from 'vitest';
+import { createMigratedTestDb } from '@/tests/helpers/db';
+import type { SqliteDatabase } from '@/lib/db/sqlite';
+import { UserRepository } from '@/repositories/UserRepository';
+import { ProjectRepository } from '@/repositories/ProjectRepository';
+import { ProjectMemberRepository } from '@/repositories/ProjectMemberRepository';
+import { BoardRepository } from '@/repositories/BoardRepository';
+import { NotificationRepository } from '@/repositories/NotificationRepository';
+import { ActivityLogRepository } from '@/repositories/ActivityLogRepository';
+import { NotificationService } from '@/services/NotificationService';
+import { ActivityLogService } from '@/services/ActivityLogService';
+import { BoardService } from '@/services/BoardService';
+import { ForbiddenError, NotFoundError, ValidationError } from '@/lib/errors';
+
+function makeService(db: SqliteDatabase) {
+  return new BoardService(
+    new BoardRepository(db),
+    new ProjectMemberRepository(db),
+    new NotificationService(new NotificationRepository(db)),
+    new ActivityLogService(new ActivityLogRepository(db))
+  );
+}
+
+describe('BoardService', () => {
+  let db: SqliteDatabase;
+  let service: BoardService;
+  let projectId: number;
+  let authorId: number;
+  let memberId: number;
+  let outsiderId: number;
+
+  beforeEach(() => {
+    db = createMigratedTestDb();
+    service = makeService(db);
+    const users = new UserRepository(db);
+    authorId = users.create({
+      name: 'Author',
+      email: 'a@example.com',
+      passwordHash: 'h',
+    }).id;
+    memberId = users.create({
+      name: 'Member',
+      email: 'm@example.com',
+      passwordHash: 'h',
+    }).id;
+    outsiderId = users.create({
+      name: 'Outsider',
+      email: 'o@example.com',
+      passwordHash: 'h',
+    }).id;
+    projectId = new ProjectRepository(db).create({
+      name: 'P',
+      ownerId: authorId,
+    }).id;
+    const members = new ProjectMemberRepository(db);
+    members.add(projectId, authorId, 'admin');
+    members.add(projectId, memberId, 'member');
+  });
+
+  afterEach(() => db.close());
+
+  it('allows a member to create a thread and records activity', () => {
+    const thread = service.createThread(memberId, projectId, {
+      title: 'T',
+      bodyMd: 'body',
+      category: 'notice',
+    });
+    expect(thread.title).toBe('T');
+
+    const logs = new ActivityLogRepository(db).findByProject(projectId);
+    expect(logs.items.some((l) => l.action === 'board_posted')).toBe(true);
+  });
+
+  it('forbids a non-member from creating a thread', () => {
+    expect(() =>
+      service.createThread(outsiderId, projectId, { title: 'T', bodyMd: 'b' })
+    ).toThrow(ForbiddenError);
+  });
+
+  it('rejects an empty title', () => {
+    expect(() =>
+      service.createThread(memberId, projectId, { title: '  ', bodyMd: 'b' })
+    ).toThrow(ValidationError);
+  });
+
+  it('only the author or admin can edit/delete a thread', () => {
+    const users = new UserRepository(db);
+    const member2 = users.create({
+      name: 'Member2',
+      email: 'm2@example.com',
+      passwordHash: 'h',
+    }).id;
+    new ProjectMemberRepository(db).add(projectId, member2, 'member');
+
+    const thread = service.createThread(memberId, projectId, {
+      title: 'T',
+      bodyMd: 'b',
+    });
+    // 非作者かつ非管理者は編集不可
+    expect(() =>
+      service.updateThread(member2, thread.id, { title: 'X' })
+    ).toThrow(ForbiddenError);
+    // 管理者は編集可
+    expect(() =>
+      service.updateThread(authorId, thread.id, { title: 'X' })
+    ).not.toThrow();
+    // 作者自身は編集可
+    expect(() =>
+      service.updateThread(memberId, thread.id, { title: 'Y' })
+    ).not.toThrow();
+    // 外部者はアクセス不可
+    expect(() => service.getThread(outsiderId, thread.id)).toThrow(
+      ForbiddenError
+    );
+  });
+
+  it('creates a comment, notifies the thread author (not the commenter), and logs activity', () => {
+    const thread = service.createThread(authorId, projectId, {
+      title: 'T',
+      bodyMd: 'b',
+    });
+    const comment = service.createComment(memberId, thread.id, 'nice post');
+
+    expect(comment.bodyMd).toBe('nice post');
+    // 通知はスレッド投稿者(authorId)へ
+    const notifs = new NotificationRepository(db).findUnreadByUser(authorId);
+    expect(notifs.total).toBe(1);
+    // コメントした本人には通知しない
+    expect(new NotificationRepository(db).countUnreadByUser(memberId)).toBe(0);
+    const logs = new ActivityLogRepository(db).findByProject(projectId);
+    expect(logs.items.some((l) => l.action === 'comment_added')).toBe(true);
+  });
+
+  it('does not notify when the author comments on their own thread', () => {
+    const thread = service.createThread(authorId, projectId, {
+      title: 'T',
+      bodyMd: 'b',
+    });
+    service.createComment(authorId, thread.id, 'self comment');
+    expect(new NotificationRepository(db).countUnreadByUser(authorId)).toBe(0);
+  });
+
+  it('only the comment author can edit their comment', () => {
+    const thread = service.createThread(authorId, projectId, {
+      title: 'T',
+      bodyMd: 'b',
+    });
+    const comment = service.createComment(memberId, thread.id, 'c');
+    expect(() => service.updateComment(authorId, comment.id, 'hacked')).toThrow(
+      ForbiddenError
+    );
+    expect(service.updateComment(memberId, comment.id, 'edited').bodyMd).toBe(
+      'edited'
+    );
+  });
+
+  it('admin can delete another member comment', () => {
+    const thread = service.createThread(authorId, projectId, {
+      title: 'T',
+      bodyMd: 'b',
+    });
+    const comment = service.createComment(memberId, thread.id, 'c');
+    service.deleteComment(authorId, comment.id); // authorId is admin
+    expect(() => service.listComments(memberId, thread.id)).not.toThrow();
+  });
+
+  it('throws NotFoundError for a non-existent thread', () => {
+    expect(() => service.getThread(memberId, 99999)).toThrow(NotFoundError);
+  });
+});