diff --git a/frontend/utils/buildFolderTree.test.ts b/frontend/utils/buildFolderTree.test.ts
new file mode 100644
index 0000000..6c277bb
--- /dev/null
+++ b/frontend/utils/buildFolderTree.test.ts
@@ -0,0 +1,63 @@
+/* eslint-disable @typescript-eslint/no-non-null-assertion */
+import { describe, it, expect } from 'vitest';
+import { buildFolderTree } from './buildFolderTree';
+import type { FolderType } from '@/types/folder';
+
+describe('buildFolderTree', () => {
+  it('should return an empty array when input is empty', () => {
+    expect(buildFolderTree([])).toEqual([]);
+  });
+
+  it('should build a single root node', () => {
+    const folders: FolderType[] = [
+      { id: 1, name: 'root', detail: '', projectId: 1, parentFolderId: null, createdAt: '', updatedAt: '', Cases: [] },
+    ];
+    const tree = buildFolderTree(folders);
+    expect(tree.length).toBe(1);
+    expect(tree[0].id).toBe('1');
+    expect(tree[0].children).toEqual([]);
+  });
+
+  it('should build a tree with parent and child', () => {
+    const folders: FolderType[] = [
+      { id: 1, name: 'root', detail: '', projectId: 1, parentFolderId: null, createdAt: '', updatedAt: '', Cases: [] },
+      { id: 2, name: 'child', detail: '', projectId: 1, parentFolderId: 1, createdAt: '', updatedAt: '', Cases: [] },
+    ];
+    const tree = buildFolderTree(folders);
+    expect(tree.length).toBe(1);
+    expect(tree[0].id).toBe('1');
+    expect(tree[0]!.children!.length).toBe(1);
+    expect(tree[0]!.children![0].id).toBe('2');
+  });
+
+  it('should build a tree with multiple levels', () => {
+    const folders: FolderType[] = [
+      { id: 1, name: 'root', detail: '', projectId: 1, parentFolderId: null, createdAt: '', updatedAt: '', Cases: [] },
+      { id: 2, name: 'child', detail: '', projectId: 1, parentFolderId: 1, createdAt: '', updatedAt: '', Cases: [] },
+      {
+        id: 3,
+        name: 'grandchild',
+        detail: '',
+        projectId: 1,
+        parentFolderId: 2,
+        createdAt: '',
+        updatedAt: '',
+        Cases: [],
+      },
+    ];
+    const tree = buildFolderTree(folders);
+    expect(tree.length).toBe(1);
+    expect(tree![0].children![0].children![0].id).toBe('3');
+  });
+
+  it('should build a forest if multiple roots', () => {
+    const folders: FolderType[] = [
+      { id: 1, name: 'root1', detail: '', projectId: 1, parentFolderId: null, createdAt: '', updatedAt: '', Cases: [] },
+      { id: 2, name: 'root2', detail: '', projectId: 1, parentFolderId: null, createdAt: '', updatedAt: '', Cases: [] },
+    ];
+    const tree = buildFolderTree(folders);
+    expect(tree.length).toBe(2);
+    expect(tree.map((n) => n.id)).toContain('1');
+    expect(tree.map((n) => n.id)).toContain('2');
+  });
+});