diff --git a/frontend/src/pages/PostDetailPage.tsx b/frontend/src/pages/PostDetailPage.tsx
index 062fa1e..22d9d49 100644
--- a/frontend/src/pages/PostDetailPage.tsx
+++ b/frontend/src/pages/PostDetailPage.tsx
@@ -31,10 +31,7 @@ export default ({ user }: Props) => {
         void (axios.delete (
                   `${ API_BASE_URL }/posts/${ id }/viewed`,
                   { headers: { 'X-Transfer-Code': localStorage.getItem ('user_code') || '' } })
-              .then (res => setPost (post => {
-                post.viewed = false
-                return post
-              }))
+              .then (res => setPost (post => ({ ...post, viewed: false })))
               .catch (err => toast ({ title:        '失敗……',
                                       description:  '通信に失敗しました……' })))
       }
@@ -44,10 +41,7 @@ export default ({ user }: Props) => {
                   `${ API_BASE_URL }/posts/${ id }/viewed`,
                   { },
                   { headers: { 'X-Transfer-Code': localStorage.getItem ('user_code') || '' } })
-              .then (res => setPost (post => {
-                post.viewed = true
-                return post
-              }))
+              .then (res => setPost (post => ({ ...post, viewed: true })))
               .catch (err => toast ({ title:        '失敗……',
                                       description:  '通信に失敗しました……' })))
       }