diff --git a/backend/db/migrate/20251210123200_add_unique_index_to_tag_implications.rb b/backend/db/migrate/20251210123200_add_unique_index_to_tag_implications.rb
index 0118f6c..681c1b5 100644
--- a/backend/db/migrate/20251210123200_add_unique_index_to_tag_implications.rb
+++ b/backend/db/migrate/20251210123200_add_unique_index_to_tag_implications.rb
@@ -1,7 +1,27 @@
 class AddUniqueIndexToTagImplications < ActiveRecord::Migration[8.0]
-  def change
+  def up
+    execute <<~SQL
+      DELETE
+        ti1
+      FROM
+        tag_implications ti1
+        INNER JOIN
+          tag_implications ti2
+          ON
+            ti1.tag_id = ti2.tag_id
+            AND ti1.parent_tag_id = ti2.parent_tag_id
+            AND ti1.id > ti2.id
+      ;
+    SQL
+
     add_index :tag_implications, [:tag_id, :parent_tag_id],
               unique: true,
               name: 'index_tag_implications_on_tag_id_and_parent_tag_id'
   end
+
+  def down
+    # NOTE: 重複削除は復元されなぃ．
+    remove_index :tag_implications,
+                 name: 'index_tag_implications_on_tag_id_and_parent_tag_id'
+  end
 end
diff --git a/frontend/src/components/TagDetailSidebar.tsx b/frontend/src/components/TagDetailSidebar.tsx
index 6fdcbad..da2c3b8 100644
--- a/frontend/src/components/TagDetailSidebar.tsx
+++ b/frontend/src/components/TagDetailSidebar.tsx
@@ -29,7 +29,7 @@ const renderTagTree = (
   return [self,
 	  ...(tag.children
 	      ?.sort ((a, b) => a.name < b.name ? -1 : 1)
-	      .flatMap (child => renderTagTree (child, nestLevel + 2, key)) ?? [])]
+	      .flatMap (child => renderTagTree (child, nestLevel + 1, key)) ?? [])]
 }