diff --git a/backend/src/models/block.ts b/backend/src/models/block.ts
index 3162afe..67b0b0d 100644
--- a/backend/src/models/block.ts
+++ b/backend/src/models/block.ts
@@ -89,6 +89,7 @@ export interface BlockDoc extends Document {
   ) => Promise<void>;
   deleteChild?: (this: BlockDoc, childId: string) => Promise<void>;
   deleteCascade?: (this: BlockDoc) => Promise<void>;
+  findIndexFromChildIdList?: (this: BlockDoc, childId: string) => number;
 }
 
 BlockSchema.statics.createOne = async function (
@@ -167,9 +168,7 @@ BlockSchema.methods.deleteChild = async function (
   this: BlockDoc,
   childId: string,
 ): Promise<void> {
-  const index = this.childIdList.findIndex(
-    (_childId) => _childId.toHexString() === childId,
-  );
+  const index = this.findIndexFromChildIdList(childId);
   this.childIdList.splice(index, 1);
   await this.save();
 };
@@ -185,4 +184,13 @@ BlockSchema.methods.deleteCascade = async function (
   await Block.findByIdAndDelete(this.id).exec();
 };
 
+BlockSchema.methods.findIndexFromChildIdList = function (
+  this: BlockDoc,
+  childId: string,
+): number {
+  return this.childIdList.findIndex(
+    (_childId) => _childId.toHexString() === childId,
+  );
+};
+
 export const Block = model<BlockDoc>(MODEL_NAME, BlockSchema) as BlockModel;
diff --git a/backend/src/services/block.ts b/backend/src/services/block.ts
index 390a1c7..46dd1b9 100644
--- a/backend/src/services/block.ts
+++ b/backend/src/services/block.ts
@@ -50,9 +50,7 @@ export const move = async (
   }
 
   toIndex ??= to.childIdList.length;
-  const fromIndex = from.childIdList.findIndex(
-    (_childId) => _childId.toHexString() === blockId,
-  );
+  const fromIndex = from.findIndexFromChildIdList(blockId);
   toIndex = fromIndex < toIndex ? toIndex - 1 : toIndex;
   await to.deleteChild(blockId);
   await to.setChild(block, toIndex);
@@ -78,9 +76,7 @@ export const deleteOnly = async (blockId: string): Promise<BlockDoc> => {
   }
 
   const parent = await Block.readOne(block.parentId.toHexString());
-  const index = parent.childIdList.findIndex(
-    (childId) => childId.toHexString() === blockId,
-  );
+  const index = parent.findIndexFromChildIdList(blockId);
 
   await parent.setChildren(block.childIdList, index);
   await parent.deleteChild(blockId);
diff --git a/backend/test/services/block.spec.ts b/backend/test/services/block.spec.ts
index d98964c..2d12ad5 100644
--- a/backend/test/services/block.spec.ts
+++ b/backend/test/services/block.spec.ts
@@ -59,11 +59,7 @@ describe('@services/block', () => {
 
     expect(received.value).toEqual(expected.value);
     expect(received.type).toEqual(expected.type);
-    expect(
-      parent.childIdList.findIndex(
-        (childId) => childId.toHexString() === received.id,
-      ),
-    ).toEqual(index);
+    expect(parent.findIndexFromChildIdList(received.id)).toEqual(index);
   });
 
   it('create: Duplicated error', async () => {