diff --git a/__tests__/comment.test.ts b/__tests__/comment.test.ts
new file mode 100644
index 0000000..5a14561
--- /dev/null
+++ b/__tests__/comment.test.ts
@@ -0,0 +1,55 @@
+import {
+  findPreviousComment,
+  createComment,
+  updateComment
+} from "../src/comment";
+const repo = {};
+const body = "some message";
+it("findPreviousComment", async () => {
+  const comment = {
+    user: {
+      login: "github-actions[bot]"
+    }
+  };
+  const otherComment = {
+    user: {
+      login: "some-user"
+    }
+  };
+  const octokit = {
+    issues: {
+      listComments: jest.fn(() =>
+        Promise.resolve({
+          data: [otherComment, comment]
+        })
+      )
+    }
+  };
+
+  expect(await findPreviousComment(octokit, repo, 123)).toBe(comment);
+  expect(octokit.issues.listComments).toBeCalledWith({ issue_number: 123 });
+});
+it("updateComment", async () => {
+  const octokit = {
+    issues: {
+      updateComment: jest.fn(() => Promise.resolve())
+    }
+  };
+  expect(await updateComment(octokit, repo, 456, body)).toBeUndefined();
+  expect(octokit.issues.updateComment).toBeCalledWith({
+    comment_id: 456,
+    body
+  });
+});
+it("createComment", async () => {
+  const octokit = {
+    issues: {
+      createComment: jest.fn(() => Promise.resolve())
+    }
+  };
+  expect(await createComment(octokit, repo, 456, body)).toBeUndefined();
+  expect(octokit.issues.createComment).toBeCalledWith({
+    issue_number: 456,
+    body
+  });
+});
diff --git a/__tests__/main.test.ts b/__tests__/main.test.ts
deleted file mode 100644
index 34fbeb9..0000000
--- a/__tests__/main.test.ts
+++ /dev/null
@@ -1,2 +0,0 @@
-test("create a comment when no comment", async () => {});
-test("update a comment when comment is exists", async () => {});