diff --git a/atcodertools/client/atcoder.py b/atcodertools/client/atcoder.py
index aa37c767..569c588b 100644
--- a/atcodertools/client/atcoder.py
+++ b/atcodertools/client/atcoder.py
@@ -4,6 +4,7 @@
 import warnings
 from http.cookiejar import LWPCookieJar
 from typing import List, Optional, Tuple, Union
+from urllib3.util.retry import Retry
 
 import requests
 from bs4 import BeautifulSoup
@@ -66,7 +67,13 @@ def default_credential_supplier() -> Tuple[str, str]:
 class AtCoderClient(metaclass=Singleton):
 
     def __init__(self):
-        self._session = requests.Session()
+        session = requests.Session()
+        retries = Retry(total=3,
+                        backoff_factor=0.5,
+                        status_forcelist=[status for status in range(400, 600)])
+        session.mount("https://",
+                      requests.adapters.HTTPAdapter(max_retries=retries))
+        self._session = session
 
     def check_logging_in(self):
         private_url = "https://atcoder.jp/home"
@@ -110,10 +117,11 @@ def login(self,
             save_cookie(self._session)
 
     def download_problem_list(self, contest: Contest) -> List[Problem]:
-        resp = self._request(contest.get_problem_list_url())
-        soup = BeautifulSoup(resp.text, "html.parser")
-        if resp.status_code == 404:
+        try:
+            resp = self._request(contest.get_problem_list_url())
+        except requests.exceptions.RetryError:
             raise PageNotFoundError
+        soup = BeautifulSoup(resp.text, "html.parser")
         res = []
         for tag in soup.find('table').select('tr')[1::]:
             tag = tag.find("a")
diff --git a/atcodertools/tools/envgen.py b/atcodertools/tools/envgen.py
index 33ba31bd..dd314e29 100755
--- a/atcodertools/tools/envgen.py
+++ b/atcodertools/tools/envgen.py
@@ -5,7 +5,6 @@
 import sys
 import traceback
 from multiprocessing import Pool, cpu_count
-import time
 from typing import Tuple
 
 from colorama import Fore
@@ -175,24 +174,12 @@ def func(argv: Tuple[AtCoderClient, Problem, Config]):
 
 def prepare_contest(atcoder_client: AtCoderClient,
                     contest_id: str,
-                    config: Config,
-                    retry_delay_secs: float = 1.5,
-                    retry_max_delay_secs: float = 60,
-                    retry_max_tries: int = 10):
-    attempt_count = 1
-    while True:
-        try:
-            problem_list = atcoder_client.download_problem_list(
-                Contest(contest_id=contest_id))
-            break
-        except PageNotFoundError:
-            if 0 < retry_max_tries < attempt_count:
-                raise EnvironmentInitializationError
-            logger.warning(
-                "Failed to fetch. Will retry in {} seconds. (Attempt {})".format(retry_delay_secs, attempt_count))
-            time.sleep(retry_delay_secs)
-            retry_delay_secs = min(retry_delay_secs * 2, retry_max_delay_secs)
-            attempt_count += 1
+                    config: Config):
+    try:
+        problem_list = atcoder_client.download_problem_list(
+            Contest(contest_id=contest_id))
+    except PageNotFoundError:
+        raise EnvironmentInitializationError
 
     tasks = [(atcoder_client,
               problem,
diff --git a/tests/test_envgen.py b/tests/test_envgen.py
index 06b9a5c9..9fbc3c7b 100755
--- a/tests/test_envgen.py
+++ b/tests/test_envgen.py
@@ -121,9 +121,9 @@ def test_skip_existing_problems(self):
 
         self.assertDirectoriesEqual(answer_data_dir_path, self.temp_dir)
 
-    @mock.patch('time.sleep')
-    def test_prepare_contest_aborts_after_max_retry_attempts(self, mock_sleep):
+    def test_prepare_contest_aborts_after_max_retry_attempts(self):
         mock_client = mock.Mock(spec=AtCoderClient)
+        # PageNotFoundError is thrown when requests.exceptions.RetryError occurs
         mock_client.download_problem_list.side_effect = PageNotFoundError
         self.assertRaises(
             EnvironmentInitializationError,
@@ -141,17 +141,6 @@ def test_prepare_contest_aborts_after_max_retry_attempts(self, mock_sleep):
                     out_example_format="output_{}.txt"
                 ))
         )
-        self.assertEqual(mock_sleep.call_count, 10)
-        mock_sleep.assert_has_calls([mock.call(1.5),
-                                     mock.call(3.0),
-                                     mock.call(6.0),
-                                     mock.call(12.0),
-                                     mock.call(24.0),
-                                     mock.call(48.0),
-                                     mock.call(60.0),
-                                     mock.call(60.0),
-                                     mock.call(60.0),
-                                     mock.call(60.0)])
 
     def assertDirectoriesEqual(self, expected_dir_path, dir_path):
         files1 = get_all_rel_file_paths(expected_dir_path)