diff --git a/index.html b/index.html
index bfc4425..b9e1d7b 100644
--- a/index.html
+++ b/index.html
@@ -9,6 +9,11 @@
     <script src="https://cdn.tailwindcss.com"></script>
     <!-- Lucide Icons -->
     <script src="https://unpkg.com/lucide@latest"></script>
+    <!-- Marked.js for Markdown parsing -->
+    <script src="https://cdn.jsdelivr.net/npm/marked/marked.min.js"></script>
+    <!-- Highlight.js for syntax highlighting -->
+    <link rel="stylesheet" href="https://cdnjs.cloudflare.com/ajax/libs/highlight.js/11.9.0/styles/github-dark.min.css">
+    <script src="https://cdnjs.cloudflare.com/ajax/libs/highlight.js/11.9.0/highlight.min.js"></script>
     <style>
         .scroll-smooth {
             scroll-behavior: smooth;
@@ -47,6 +52,70 @@
                 transform: translateY(0);
             }
         }
+
+        /* Message content styling */
+        .message-content {
+            overflow-wrap: break-word;
+            word-wrap: break-word;
+            word-break: break-word;
+        }
+
+        .message-content pre {
+            overflow-x: auto;
+            margin: 0.5rem 0;
+            padding: 1rem;
+            background-color: #1f2937;
+            border-radius: 0.5rem;
+            border: 1px solid #374151;
+        }
+
+        .message-content code {
+            font-family: 'Courier New', Courier, monospace;
+            font-size: 0.875rem;
+        }
+
+        .message-content pre code {
+            background: transparent;
+            padding: 0;
+            border-radius: 0;
+        }
+
+        .message-content p {
+            margin: 0.5rem 0;
+        }
+
+        .message-content p:first-child {
+            margin-top: 0;
+        }
+
+        .message-content p:last-child {
+            margin-bottom: 0;
+        }
+
+        .message-content ul,
+        .message-content ol {
+            margin: 0.5rem 0;
+            padding-left: 1.5rem;
+        }
+
+        .message-content li {
+            margin: 0.25rem 0;
+        }
+
+        .message-content blockquote {
+            border-left: 3px solid #4b5563;
+            padding-left: 1rem;
+            margin: 0.5rem 0;
+            color: #9ca3af;
+        }
+
+        /* Inline code */
+        .message-content :not(pre)>code {
+            background-color: #374151;
+            padding: 0.125rem 0.375rem;
+            border-radius: 0.25rem;
+            font-size: 0.875rem;
+        }
     </style>
 </head>
 
@@ -109,7 +178,7 @@
             <div
                 class="relative flex items-end gap-2 bg-gray-700 border border-gray-600 rounded-2xl p-1.5 focus-within:ring-2 focus-within:ring-indigo-500 focus-within:bg-gray-700 transition-all shadow-inner">
                 <textarea id="user-input" placeholder="Type your message..." rows="1"
-                    class="flex-1 bg-transparent border-none focus:ring-0 text-sm py-2 px-3 resize-none max-h-40 min-h-[40px] outline-none text-gray-100 placeholder-gray-400"></textarea>
+                    class="flex-1 bg-transparent border-none focus:ring-0 text-sm py-2 px-3 resize-none max-h-60 min-h-[40px] outline-none text-gray-100 placeholder-gray-400"></textarea>
                 <button id="send-btn" class="p-2.5 rounded-xl transition-all text-gray-300 disabled:cursor-not-allowed"
                     disabled>
                     <i data-lucide="send" id="send-icon" class="w-5 h-5"></i>
@@ -144,30 +213,74 @@
 
         modelLabel.textContent = `Model: ${MODEL}`;
 
+        // Configure marked.js
+        marked.setOptions({
+            breaks: true,
+            gfm: true,
+            headerIds: false,
+            mangle: false
+        });
+
         // アイコンの初期化
         function initIcons() {
             lucide.createIcons();
         }
 
+        // Format message content
+        function formatMessage(content, isUser) {
+            if (isUser) {
+                // User messages: preserve formatting but allow line breaks
+                return `<div class="message-content whitespace-pre-wrap">${escapeHtml(content)}</div>`;
+            } else {
+                // AI messages: parse as Markdown
+                try {
+                    const html = marked.parse(content);
+                    return `<div class="message-content">${html}</div>`;
+                } catch (e) {
+                    console.error('Markdown parsing error:', e);
+                    return `<div class="message-content whitespace-pre-wrap">${escapeHtml(content)}</div>`;
+                }
+            }
+        }
+
+        // Escape HTML to prevent XSS
+        function escapeHtml(text) {
+            const map = {
+                '&': '&amp;',
+                '<': '&lt;',
+                '>': '&gt;',
+                '"': '&quot;',
+                "'": '&#039;'
+            };
+            return text.replace(/[&<>"']/g, m => map[m]);
+        }
+
         // メッセージのレンダリング
         function renderMessages() {
             container.innerHTML = '';
             messages.forEach((msg) => {
                 const isUser = msg.role === 'user';
-                const messageHtml = `
-                    <div class="flex gap-4 ${isUser ? 'flex-row-reverse' : 'flex-row'} message-animation">
-                        <div class="flex-shrink-0 w-8 h-8 rounded-full flex items-center justify-center shadow-sm ${isUser ? 'bg-indigo-600' : 'bg-gray-700 border border-gray-600'}">
-                            <i data-lucide="${isUser ? 'user' : 'bot'}" class="${isUser ? 'text-white' : 'text-indigo-400'} w-4 h-4"></i>
-                        </div>
-                        <div class="flex flex-col max-w-[85%] ${isUser ? 'items-end' : 'items-start'}">
-                            <div class="px-4 py-2.5 rounded-2xl shadow-sm text-sm ${isUser ? 'bg-indigo-600 text-white rounded-tr-none' : 'bg-gray-800 text-gray-100 border border-gray-700 rounded-tl-none'}">
-                                <p class="whitespace-pre-wrap break-words">${msg.content}</p>
-                            </div>
+                const formattedContent = formatMessage(msg.content, isUser);
+                const messageDiv = document.createElement('div');
+                messageDiv.className = `flex gap-4 ${isUser ? 'flex-row-reverse' : 'flex-row'} message-animation`;
+                messageDiv.innerHTML = `
+                    <div class="flex-shrink-0 w-8 h-8 rounded-full flex items-center justify-center shadow-sm ${isUser ? 'bg-indigo-600' : 'bg-gray-700 border border-gray-600'}">
+                        <i data-lucide="${isUser ? 'user' : 'bot'}" class="${isUser ? 'text-white' : 'text-indigo-400'} w-4 h-4"></i>
+                    </div>
+                    <div class="flex flex-col flex-1 ${isUser ? 'items-end' : 'items-start'}">
+                        <div class="px-4 py-2.5 rounded-2xl shadow-sm text-sm max-w-full overflow-hidden ${isUser ? 'bg-indigo-600 text-white rounded-tr-none' : 'bg-gray-800 text-gray-100 border border-gray-700 rounded-tl-none'}">
+                            ${formattedContent}
                         </div>
                     </div>
                 `;
-                container.insertAdjacentHTML('beforeend', messageHtml);
+                container.appendChild(messageDiv);
             });
+
+            // Apply syntax highlighting to code blocks
+            document.querySelectorAll('pre code').forEach((block) => {
+                hljs.highlightElement(block);
+            });
+
             initIcons();
             scrollToBottom();
         }