diff --git a/README.md b/README.md
index d68e9556..955844d4 100644
--- a/README.md
+++ b/README.md
@@ -29,23 +29,15 @@ Listening on http://localhost:8080/
 Hit Ctrl-C to quit.
 ```
 
-<details>
-
-<summary>Running optuna-dashboard with Gunicorn</summary>
-
-optuna-dashboard uses [wsgiref](https://docs.python.org/3/library/wsgiref.html) module
-which is provided as a Python standard library. But it has not been reviewed for security
-issues, so not suitable for the production use. You can run optuna-dashboard with Gunicorn
-more secure and/or more fast.
+Note that optuna-dashboard uses [wsgiref](https://docs.python.org/3/library/wsgiref.html) module by default.
+Although it requires no additional dependencies, it is NOT suitable for the production use.
+You can use [Gunicorn](https://gunicorn.org/) via "--server gunicorn" option.
 
 ```console
 $ pip install gunicorn
 $ optuna-dashboard sqlite:///db.sqlite3 --server gunicorn
 ```
 
-</details>
-
-
 <details>
 
 <summary>More command line options</summary>
@@ -75,7 +67,7 @@ optional arguments:
 
 <summary>Python Interface</summary>
 
-**`run_server(storage: Union[str, BaseStorage], host: str = 'localhost', port: int = 8080) -> NoReturn`**
+**`run_server(storage: Union[str, BaseStorage], host: str = 'localhost', port: int = 8080) -> None`**
 
 Start running optuna-dashboard and blocks until the server terminates.
 This function uses wsgiref module which is not intended for the production use.
diff --git a/optuna_dashboard/_app.py b/optuna_dashboard/_app.py
index cafdff30..47320fb8 100644
--- a/optuna_dashboard/_app.py
+++ b/optuna_dashboard/_app.py
@@ -12,7 +12,6 @@ from typing import Callable
 from typing import cast
 from typing import Dict
 from typing import List
-from typing import NoReturn
 from typing import Optional
 from typing import TypeVar
 from typing import Union
@@ -294,9 +293,9 @@ def get_storage(storage: Union[str, BaseStorage]) -> BaseStorage:
     return storage
 
 
-def run_server(  # type: ignore
+def run_server(
     storage: Union[str, BaseStorage], host: str = "localhost", port: int = 8080
-) -> NoReturn:
+) -> None:
     """Start running optuna-dashboard and blocks until the server terminates.
     This function uses wsgiref module which is not intended for the production
     use. If you want to run optuna-dashboard more secure and/or more fast,
diff --git a/optuna_dashboard/_cli.py b/optuna_dashboard/_cli.py
index 7812de0d..c97ca482 100644
--- a/optuna_dashboard/_cli.py
+++ b/optuna_dashboard/_cli.py
@@ -1,6 +1,5 @@
 import argparse
 import os
-from typing import NoReturn
 
 from bottle import Bottle
 from bottle import run
@@ -16,7 +15,7 @@ AUTO_RELOAD = os.environ.get("OPTUNA_DASHBOARD_AUTO_RELOAD") == "1"
 SERVER_CHOICES = ["wsgiref", "gunicorn"]
 
 
-def run_wsgiref(app: Bottle, host: str, port: int, quiet: bool) -> NoReturn:  # type: ignore
+def run_wsgiref(app: Bottle, host: str, port: int, quiet: bool) -> None:
     run(
         app,
         host=host,
@@ -27,12 +26,12 @@ def run_wsgiref(app: Bottle, host: str, port: int, quiet: bool) -> NoReturn:  #
     )
 
 
-def run_gunicorn(app: Bottle, host: str, port: int, quiet: bool) -> NoReturn:  # type: ignore
+def run_gunicorn(app: Bottle, host: str, port: int, quiet: bool) -> None:
     # See https://docs.gunicorn.org/en/latest/custom.html
 
     from gunicorn.app.base import BaseApplication
 
-    class _Application(BaseApplication):
+    class Application(BaseApplication):
         def load_config(self) -> None:
             self.cfg.set("bind", f"{host}:{port}")
             if quiet:
@@ -41,10 +40,10 @@ def run_gunicorn(app: Bottle, host: str, port: int, quiet: bool) -> NoReturn:  #
         def load(self) -> Bottle:
             return app
 
-    _Application().run()
+    Application().run()
 
 
-def main() -> NoReturn:
+def main() -> None:
     parser = argparse.ArgumentParser(description="Real-time dashboard for Optuna.")
     parser.add_argument("storage", help="DB URL (e.g. sqlite:///example.db)", type=str)
     parser.add_argument(