diff --git a/backend/app/models/tag.rb b/backend/app/models/tag.rb
index d083dc6..21e20b4 100644
--- a/backend/app/models/tag.rb
+++ b/backend/app/models/tag.rb
@@ -148,25 +148,23 @@ class Tag < ApplicationRecord
     target_tag => Tag
 
     Tag.transaction do
-      Array(source_tags).compact.uniq.each do |st|
-        st => Tag
-
-        next if st == target_tag
-
-        st.post_tags.find_each do |pt|
-          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, updated_at: Time.current)
-          else
-            pt.update!(tag: target_tag)
+      Array(source_tags).compact.uniq.each do |source_tag|
+        source_tag => Tag
+
+        next if source_tag == target_tag
+
+        source_tag.post_tags.kept.find_each do |source_pt|
+          post_id = source_pt.post_id
+          source_pt.discard_by!(nil)
+          unless PostTag.kept.exists?(post_id:, tag: target_tag)
+            PostTag.create!(post_id:, tag: target_tag)
           end
         end
 
-        tag_name = st.tag_name
-        st.discard!
-        tag_name.reload
-        tag_name.update!(canonical: target_tag.tag_name)
+        source_tag_name = source_tag.tag_name
+        source_tag.discard!
+        source_tag_name.reload
+        source_tag_name.update!(canonical: target_tag.tag_name)
       end
 
       # 投稿件数を再集計
diff --git a/backend/spec/models/tag_spec.rb b/backend/spec/models/tag_spec.rb
index 389f1a1..c2947fa 100644
--- a/backend/spec/models/tag_spec.rb
+++ b/backend/spec/models/tag_spec.rb
@@ -10,11 +10,18 @@ RSpec.describe Tag, type: :model do
     context 'when merging a simple source tag' do
       let!(:source_post_tag) { PostTag.create!(post: post_record, tag: source_tag) }
 
-      it 'moves the post_tag, deletes the source tag, and aliases the source tag_name' do
+      it 'discards the source post_tag, creates an active target post_tag, ' +
+         'discards the source tag, and aliases the source tag_name' do
         described_class.merge_tags!(target_tag, [source_tag])
 
-        expect(source_post_tag.reload.tag_id).to eq(target_tag.id)
-        expect(Tag.exists?(source_tag.id)).to be(false)
+        source_pt = PostTag.with_discarded.find(source_post_tag.id)
+        active_target = PostTag.kept.find_by(post_id: post_record.id, tag_id: target_tag.id)
+
+        expect(source_pt.discarded_at).to be_present
+        expect(source_pt.tag_id).to eq(source_tag.id)
+        expect(active_target).to be_present
+
+        expect(Tag.with_discarded.find(source_tag.id)).to be_discarded
         expect(source_tag.tag_name.reload.canonical_id).to eq(target_tag.tag_name_id)
       end
     end
@@ -23,15 +30,19 @@ RSpec.describe Tag, type: :model do
       let!(:target_post_tag) { PostTag.create!(post: post_record, tag: target_tag) }
       let!(:source_post_tag) { PostTag.create!(post: post_record, tag: source_tag) }
 
-      it 'discards the duplicate source post_tag and keeps one active target post_tag' do
+      it 'discards the source post_tag, keeps one active target post_tag, ' +
+         'and aliases the source tag_name' do
         described_class.merge_tags!(target_tag, [source_tag])
 
+        source_pt = PostTag.with_discarded.find(source_post_tag.id)
         active = PostTag.kept.where(post_id: post_record.id, tag_id: target_tag.id)
-        discarded_source = PostTag.with_discarded.find(source_post_tag.id)
 
+        expect(source_pt.discarded_at).to be_present
+        expect(source_pt.tag_id).to eq(source_tag.id)
         expect(active.count).to eq(1)
-        expect(discarded_source.discarded_at).to be_present
-        expect(Tag.exists?(source_tag.id)).to be(false)
+        expect(active.first.id).to eq(target_post_tag.id)
+
+        expect(Tag.with_discarded.find(source_tag.id)).to be_discarded
         expect(source_tag.tag_name.reload.canonical_id).to eq(target_tag.tag_name_id)
       end
     end
@@ -39,12 +50,17 @@ RSpec.describe Tag, type: :model do
     context 'when source_tags includes the target itself' do
       let!(:source_post_tag) { PostTag.create!(post: post_record, tag: source_tag) }
 
-      it 'ignores the target tag in source_tags' do
+      it 'ignores the target in source_tags while still merging the source tag' do
         described_class.merge_tags!(target_tag, [source_tag, target_tag])
 
-        expect(Tag.exists?(target_tag.id)).to be(true)
-        expect(Tag.exists?(source_tag.id)).to be(false)
-        expect(source_post_tag.reload.tag_id).to eq(target_tag.id)
+        source_pt = PostTag.with_discarded.find(source_post_tag.id)
+        active_target = PostTag.kept.find_by(post_id: post_record.id, tag_id: target_tag.id)
+
+        expect(Tag.find(target_tag.id)).to be_present
+        expect(Tag.with_discarded.find(source_tag.id)).to be_discarded
+        expect(source_pt.discarded_at).to be_present
+        expect(source_pt.tag_id).to eq(source_tag.id)
+        expect(active_target).to be_present
       end
     end
 
@@ -52,9 +68,10 @@ RSpec.describe Tag, type: :model do
       let!(:source_post_tag) { PostTag.create!(post: post_record, tag: source_tag) }
       let!(:wiki_page) do
         WikiPage.create!(
-            tag_name: source_tag.tag_name,
-            created_user: create_admin_user!,
-            updated_user: create_admin_user!)
+          tag_name: source_tag.tag_name,
+          created_user: create_admin_user!,
+          updated_user: create_admin_user!
+        )
       end
 
       it 'rolls back the transaction' do
@@ -62,8 +79,8 @@ RSpec.describe Tag, type: :model do
           described_class.merge_tags!(target_tag, [source_tag])
         }.to raise_error(ActiveRecord::RecordInvalid)
 
-        expect(Tag.exists?(source_tag.id)).to be(true)
-        expect(source_post_tag.reload.tag_id).to eq(source_tag.id)
+        expect(Tag.with_discarded.find(source_tag.id)).not_to be_discarded
+        expect(PostTag.kept.find(source_post_tag.id).tag_id).to eq(source_tag.id)
         expect(source_tag.tag_name.reload.canonical_id).to be_nil
       end
     end