diff --git a/frontend/src/components/PostEditForm.test.tsx b/frontend/src/components/PostEditForm.test.tsx
index 1fc58a9..2ff28b1 100644
--- a/frontend/src/components/PostEditForm.test.tsx
+++ b/frontend/src/components/PostEditForm.test.tsx
@@ -66,4 +66,27 @@ describe ('PostEditForm', () => {
     expect (onSave).toHaveBeenCalledWith (expect.objectContaining ({ versionNo: 5 }))
     expect (toastApi.toast).toHaveBeenCalledWith ({ description: '更新しました．' })
   })
+
+  it ('preserves duration while the video tag is temporarily removed', () => {
+    const post = buildPost ({
+      videoMs: 180_500,
+      tags:    [
+        buildTag ({ id: 1, name: '動画', category: 'general' }),
+        buildTag ({ id: 2, name: 'general-tag', category: 'general' }),
+      ],
+    })
+
+    render (<PostEditForm post={post} onSave={vi.fn ()}/>)
+
+    expect (screen.getByRole ('spinbutton')).toHaveValue (180.5)
+
+    const tags = screen.getAllByRole ('textbox')[2]
+    fireEvent.change (tags, { target: { value: 'general-tag' } })
+    expect (screen.queryByRole ('spinbutton')).not.toBeInTheDocument ()
+
+    fireEvent.change (tags, {
+      target: { value: '動画 general-tag' },
+    })
+    expect (screen.getByRole ('spinbutton')).toHaveValue (180.5)
+  })
 })
diff --git a/frontend/src/components/PostEditForm.tsx b/frontend/src/components/PostEditForm.tsx
index c1fdd72..744c132 100644
--- a/frontend/src/components/PostEditForm.tsx
+++ b/frontend/src/components/PostEditForm.tsx
@@ -153,11 +153,6 @@ const PostEditForm: FC<Props> = ({ post, onSave }) => {
     setDuration (videoMsToDurationValue (post.videoMs))
   }, [post])
 
-  useEffect (() => {
-    if (!(videoFlg))
-      setDuration ('')
-  }, [videoFlg])
-
   return (
       <form onSubmit={handleSubmit} className="max-w-xl pt-2 space-y-4">
 	<FieldError messages={baseErrors}/>
diff --git a/frontend/src/pages/posts/PostNewPage.test.tsx b/frontend/src/pages/posts/PostNewPage.test.tsx
index b8c909e..1402ed0 100644
--- a/frontend/src/pages/posts/PostNewPage.test.tsx
+++ b/frontend/src/pages/posts/PostNewPage.test.tsx
@@ -62,6 +62,24 @@ describe ('PostNewPage', () => {
     expect (toastApi.toast).toHaveBeenCalledWith ({ title: '投稿成功!' })
   })
 
+  it ('preserves duration while the video tag is temporarily removed', () => {
+    api.apiGet.mockResolvedValue ([])
+
+    renderWithProviders (<PostNewPage user={buildUser ({ role: 'member' })}/>)
+
+    const tags = screen.getAllByRole ('textbox')[3]
+    fireEvent.change (tags, { target: { value: '動画' } })
+    fireEvent.change (screen.getByRole ('spinbutton'), { target: { value: '180.5' } })
+
+    fireEvent.change (tags, { target: { value: 'general-tag' } })
+    expect (screen.queryByRole ('spinbutton')).not.toBeInTheDocument ()
+
+    fireEvent.change (tags, {
+      target: { value: '動画 general-tag' },
+    })
+    expect (screen.getByRole ('spinbutton')).toHaveValue (180.5)
+  })
+
   it ('shows 422 validation errors for post fields', async () => {
     api.apiGet.mockResolvedValue ([])
     api.isApiError.mockReturnValue (true)
diff --git a/frontend/src/pages/posts/PostNewPage.tsx b/frontend/src/pages/posts/PostNewPage.tsx
index 03a433c..0a99dc5 100644
--- a/frontend/src/pages/posts/PostNewPage.tsx
+++ b/frontend/src/pages/posts/PostNewPage.tsx
@@ -135,11 +135,6 @@ const PostNewPage: FC<Props> = ({ user }) => {
       fetchThumbnail ()
   }, [fetchThumbnail, thumbnailAutoFlg, url])
 
-  useEffect (() => {
-    if (!(videoFlg))
-      setDuration ('')
-  }, [videoFlg])
-
   if (!(editable))
     return <Forbidden/>