diff --git a/ui/builder.html b/ui/builder.html
index badae63..f96cc7a 100644
--- a/ui/builder.html
+++ b/ui/builder.html
@@ -3,7 +3,7 @@
 <head>
 <meta charset="UTF-8">
 <meta name="viewport" content="width=device-width, initial-scale=1.0">
-<title>CTStruct 양식 빌더</title>
+<title>CTStruct</title>
 <style>
 *,*::before,*::after{box-sizing:border-box;margin:0;padding:0}
 :root{
@@ -27,10 +27,21 @@ body{font-family:-apple-system,BlinkMacSystemFont,'Segoe UI',sans-serif;backgrou
 .hdr-btn{display:flex;align-items:center;gap:5px;font-size:12px;padding:5px 12px;border-radius:6px;cursor:pointer;border:1px solid var(--border);background:var(--surface);color:var(--muted);transition:all .15s;font-weight:500}
 .hdr-btn:hover{background:var(--surface-alt);color:var(--text);border-color:var(--border-focus)}
 
+/* ── 메인 탭 네비게이션 ───────────────────── */
+.main-nav{background:var(--surface);border-bottom:2px solid var(--border);padding:0 20px;display:flex;gap:4px;flex-shrink:0}
+.main-tab{font-size:13px;font-weight:600;padding:11px 18px;border:none;background:transparent;color:var(--muted);cursor:pointer;border-bottom:2.5px solid transparent;margin-bottom:-2px;transition:all .15s;display:flex;align-items:center;gap:6px;letter-spacing:.01em}
+.main-tab:hover:not(.active){color:var(--text)}
+.main-tab.active{color:var(--accent);border-bottom-color:var(--accent)}
+
+/* ── 탭 패널 ─────────────────────────────── */
+.tab-panel{flex:1;display:flex;overflow:hidden}
+.tab-panel.hidden{display:none!important}
+
 /* ── 레이아웃 ─────────────────────────────── */
 .app-body{display:flex;flex:1;overflow:hidden}
 .builder-panel{width:460px;min-width:340px;background:var(--surface);border-right:1px solid var(--border);display:flex;flex-direction:column;overflow:hidden}
-.panel-lbl{padding:10px 16px 8px;font-size:10px;font-weight:700;text-transform:uppercase;letter-spacing:.08em;color:var(--muted);border-bottom:1px solid var(--border);flex-shrink:0;background:var(--surface-alt)}
+.panel-lbl{padding:8px 12px 7px;font-size:10px;font-weight:700;text-transform:uppercase;letter-spacing:.08em;color:var(--muted);border-bottom:1px solid var(--border);flex-shrink:0;background:var(--surface-alt);display:flex;align-items:center;gap:6px}
+.panel-lbl-title{flex:1}
 .builder-scroll{flex:1;overflow-y:auto;padding:12px}
 .builder-scroll::-webkit-scrollbar{width:5px}
 .builder-scroll::-webkit-scrollbar-thumb{background:var(--border);border-radius:3px}
@@ -154,16 +165,23 @@ input,textarea,button,select{font-family:inherit}
 
 <header class="app-header">
   <span style="font-size:18px">🏗️</span>
-  <h1>CTStruct 양식 빌더</h1>
-  <span class="badge">v0.4</span>
-  <div class="hdr-spacer"></div>
-  <button class="hdr-btn" onclick="openImport()">📥 불러오기</button>
-  <button class="hdr-btn" onclick="newDoc()">🗒 새 문서</button>
+  <h1>CTStruct</h1>
+  <span class="badge">v0.5</span>
 </header>
 
+<nav class="main-nav">
+  <button class="main-tab active" id="mtab-builder" onclick="switchTab('builder')">✏️ 양식 만들기</button>
+  <button class="main-tab"        id="mtab-filler"  onclick="switchTab('filler')">📋 내용 채우기</button>
+</nav>
+
+<div id="panel-builder" class="tab-panel">
 <div class="app-body">
   <div class="builder-panel">
-    <div class="panel-lbl">✏️ 편집기</div>
+    <div class="panel-lbl">
+      <span class="panel-lbl-title">✏️ 편집기</span>
+      <button class="hdr-btn" onclick="openImport()">📥 불러오기</button>
+      <button class="hdr-btn" onclick="newDoc()">🗒 새 문서</button>
+    </div>
     <div class="builder-scroll">
       <div class="root-row">
         <span class="root-lbl">문서</span>
@@ -192,6 +210,15 @@ input,textarea,button,select{font-family:inherit}
     </div>
   </div>
 </div>
+</div><!-- /panel-builder -->
+
+<div id="panel-filler" class="tab-panel hidden">
+  <div style="flex:1;display:flex;align-items:center;justify-content:center;flex-direction:column;gap:12px;color:var(--muted)">
+    <span style="font-size:48px">📋</span>
+    <p style="font-size:15px;font-weight:600">내용 채우기</p>
+    <p style="font-size:12px">Step 3에서 구현 예정</p>
+  </div>
+</div>
 
 <div class="overlay" id="importModal">
   <div class="modal">
@@ -868,6 +895,17 @@ function newDoc(){
   S={titleMark:'✅',title:'',nodes:[]}; refresh();
 }
 
+// ════════════════════════════════════════════════════════
+// 탭 전환
+// ════════════════════════════════════════════════════════
+const TAB_IDS = ['builder', 'filler'];
+function switchTab(name){
+  TAB_IDS.forEach(id => {
+    document.getElementById('panel-'+id).classList.toggle('hidden', id !== name);
+    document.getElementById('mtab-'+id).classList.toggle('active', id === name);
+  });
+}
+
 // ════════════════════════════════════════════════════════
 // 초기화
 // ════════════════════════════════════════════════════════
