diff --git a/src/main/ipc/files.js b/src/main/ipc/files.js
index 83ecd97..c249d03 100644
--- a/src/main/ipc/files.js
+++ b/src/main/ipc/files.js
@@ -95,7 +95,6 @@ function register() {
   ipcMain.handle('rename-files', async (_event, { folderPath, files, prefix, digits, postfix = '' }) => {
     const errors = [];
     const pad = (n) => String(n + 1).padStart(digits, '0');
-    const suffix = postfix ? `_${postfix}` : '';
 
     // 1단계: 임시 이름으로 이동 (충돌 방지)
     const tmpMap = [];
@@ -114,7 +113,7 @@ function register() {
     // 2단계: 최종 이름으로 이동
     const renamed = [];
     for (const { tmp, ext, idx } of tmpMap) {
-      const newName = `${prefix}_${pad(idx)}${suffix}${ext}`;
+      const newName = `${prefix}${pad(idx)}${postfix}${ext}`;
       const dest = path.join(folderPath, newName);
       try {
         fs.renameSync(tmp, dest);
diff --git a/src/renderer/folder-controller.js b/src/renderer/folder-controller.js
index 3ebe748..64a43d8 100644
--- a/src/renderer/folder-controller.js
+++ b/src/renderer/folder-controller.js
@@ -69,7 +69,7 @@ export function createFolderController({
       setStatus(`${state.files.length}개 파일 로드됨`, true);
 
       const folderName = folder.replace(/\\/g, '/').split('/').pop();
-      if (folderName) inputPrefix.value = folderName;
+      if (folderName) inputPrefix.value = `${folderName}_`;
 
       renderGrid();
       updatePreview();
diff --git a/src/renderer/index.html b/src/renderer/index.html
index 5fba393..6297e53 100644
--- a/src/renderer/index.html
+++ b/src/renderer/index.html
@@ -41,14 +41,12 @@
 
     <form id="format-bar">
       <label for="input-prefix">파일명 포맷</label>
-      <input type="text" id="input-prefix" value="photo" placeholder="접두사" />
-      <span class="format-sep">_</span>
+      <input type="text" id="input-prefix" value="photo_" placeholder="접두사" />
       <input type="number" id="input-digits" value="4" min="1" max="6" />
       <span class="format-sep">자리 순번</span>
-      <span class="format-sep">_</span>
-      <input type="text" id="input-postfix" maxlength="3" size="3" placeholder="접미사" />
+      <input type="text" id="input-postfix" value="_" maxlength="3" size="3" placeholder="접미사" />
       <span class="format-arrow">→</span>
-      <span id="preview-name">photo_001.jpg</span>
+      <span id="preview-name">photo_0001_.jpg</span>
       <span id="sel-count"></span>
       <span id="file-count">0개 파일</span>
     </form>
diff --git a/src/renderer/renderer.js b/src/renderer/renderer.js
index aebf236..9df00a3 100644
--- a/src/renderer/renderer.js
+++ b/src/renderer/renderer.js
@@ -146,11 +146,10 @@ initKeyboardShortcuts({
 
 // ── 포맷 미리보기 ─────────────────────────────────
 function updatePreview() {
-  const prefix = inputPrefix.value.trim() || 'photo';
+  const prefix = inputPrefix.value.trim() || 'photo_';
   const digits = clampDigits(inputDigits.value);
   const postfix = inputPostfix.value.trim();
   const ext    = state.files.length ? getExtension(state.files[0].name) : 'jpg';
-  const suffix = postfix ? `_${postfix}` : '';
-  previewName.textContent = `${prefix}_${'1'.padStart(digits, '0')}${suffix}.${ext}`;
+  previewName.textContent = `${prefix}${'1'.padStart(digits, '0')}${postfix}.${ext}`;
   updateNewNameLabels();
 }
diff --git a/src/renderer/utils/file-names.js b/src/renderer/utils/file-names.js
index 95e7096..26dd6a5 100644
--- a/src/renderer/utils/file-names.js
+++ b/src/renderer/utils/file-names.js
@@ -9,14 +9,12 @@ export function clampDigits(value, fallback = 3) {
 
 export function buildSequentialName(fileName, index, { prefix, digits, postfix = '' }) {
   const ext = getExtension(fileName);
-  const sequence = `${prefix}_${String(index + 1).padStart(digits, '0')}`;
-  const suffix = postfix ? `_${postfix}` : '';
-  return `${sequence}${suffix}.${ext}`;
+  return `${prefix}${String(index + 1).padStart(digits, '0')}${postfix}.${ext}`;
 }
 
 export function getNameFormat(prefixInput, digitsInput, digitsFallback = 3, postfixInput = null) {
   return {
-    prefix: prefixInput.value.trim() || 'photo',
+    prefix: prefixInput.value.trim() || 'photo_',
     digits: clampDigits(digitsInput.value, digitsFallback),
     postfix: postfixInput?.value.trim() || '',
   };