diff --git a/backend/app/models/tag.rb b/backend/app/models/tag.rb
index fd49e10..4e6f133 100644
--- a/backend/app/models/tag.rb
+++ b/backend/app/models/tag.rb
@@ -153,7 +153,7 @@ class Tag < ApplicationRecord
           if PostTag.kept.exists?(post_id: pt.post_id, tag_id: target_tag.id)
             pt.discard_by!(nil)
             # discard 後の update! は禁止なので DB を直に更新
-            pt.update_columns(tag_id: target_tag.id)
+            pt.update_columns(tag_id: target_tag.id, updated_at: Time.current)
           else
             pt.update!(tag: target_tag)
           end