diff --git a/nvchecker/__main__.py b/nvchecker/__main__.py
index 602e7b9..76d0c59 100755
--- a/nvchecker/__main__.py
+++ b/nvchecker/__main__.py
@@ -44,11 +44,11 @@ def main() -> None:
   if options.proxy is not None:
     ctx_proxy.set(options.proxy)
 
-  token_q = core.token_queue(options.max_concurrency)
+  task_sem = asyncio.Semaphore(options.max_concurrency)
   result_q: asyncio.Queue[RawResult] = asyncio.Queue()
   try:
     futures = core.dispatch(
-      entries, token_q, result_q,
+      entries, task_sem, result_q,
       keymanager, args.tries,
     )
   except ModuleNotFoundError as e:
diff --git a/nvchecker/core.py b/nvchecker/core.py
index 8b320d5..0dec714 100644
--- a/nvchecker/core.py
+++ b/nvchecker/core.py
@@ -193,17 +193,9 @@ def load_file(
   return cast(Entries, config), Options(
     ver_files, max_concurrency, proxy, keymanager)
 
-def token_queue(maxsize: int) -> Queue[bool]:
-  token_q: Queue[bool] = Queue(maxsize=maxsize)
-
-  for _ in range(maxsize):
-    token_q.put_nowait(True)
-
-  return token_q
-
 def dispatch(
   entries: Entries,
-  token_q: Queue[bool],
+  task_sem: asyncio.Semaphore,
   result_q: Queue[RawResult],
   keymanager: KeyManager,
   tries: int,
@@ -232,7 +224,7 @@ def dispatch(
     ctx = root_ctx.copy()
     worker = ctx.run(
       worker_cls,
-      token_q, result_q, tasks, keymanager,
+      task_sem, result_q, tasks, keymanager,
     )
     if worker_cls is FunctionWorker:
       func = mod.get_version # type: ignore
diff --git a/nvchecker/util.py b/nvchecker/util.py
index d35ff70..7c167af 100644
--- a/nvchecker/util.py
+++ b/nvchecker/util.py
@@ -5,9 +5,8 @@ from __future__ import annotations
 
 import asyncio
 from asyncio import Queue
-import contextlib
 from typing import (
-  Dict, Optional, List, AsyncGenerator, NamedTuple, Union,
+  Dict, Optional, List, NamedTuple, Union,
   Any, Tuple, Callable, Coroutine, Hashable,
   TYPE_CHECKING,
 )
@@ -72,10 +71,10 @@ class Result(NamedTuple):
 class BaseWorker:
   '''The base class for defining `Worker` classes for source plugins.
 
-  .. py:attribute:: token_q
-      :type: Queue[bool]
+  .. py:attribute:: task_sem
+      :type: asyncio.Semaphore
 
-      This is the rate-limiting queue. Workers should obtain one token before doing one unit of work.
+      This is the rate-limiting semaphore. Workers should acquire it while doing one unit of work.
 
   .. py:attribute:: result_q
       :type: Queue[RawResult]
@@ -96,28 +95,16 @@ class BaseWorker:
   '''
   def __init__(
     self,
-    token_q: Queue[bool],
+    task_sem: asyncio.Semaphore,
     result_q: Queue[RawResult],
     tasks: List[Tuple[str, Entry]],
     keymanager: KeyManager,
   ) -> None:
-    self.token_q = token_q
+    self.task_sem = task_sem
     self.result_q = result_q
     self.keymanager = keymanager
     self.tasks = tasks
 
-  @contextlib.asynccontextmanager
-  async def acquire_token(self) -> AsyncGenerator[None, None]:
-    '''A context manager to obtain a token from the `token_q` on entrance and
-    release it on exit.'''
-    token = await self.token_q.get()
-    logger.debug('got token')
-    try:
-      yield
-    finally:
-      await self.token_q.put(token)
-      logger.debug('return token')
-
   @abc.abstractmethod
   async def run(self) -> None:
     '''Run the `tasks`. Subclasses should implement this method.'''
@@ -227,7 +214,7 @@ class FunctionWorker(BaseWorker):
       ctx_ua.set(ua)
 
     try:
-      async with self.acquire_token():
+      async with self.task_sem:
         version = await self.func(
           name, entry,
           cache = self.cache,
diff --git a/nvchecker_source/apt.py b/nvchecker_source/apt.py
index 147e6d1..b1bedb9 100644
--- a/nvchecker_source/apt.py
+++ b/nvchecker_source/apt.py
@@ -20,8 +20,6 @@ def _decompress_data(url: str, data: bytes) -> str:
   elif url.endswith(".gz"):
     import gzip
     data = gzip.decompress(data)
-  else:
-    raise NotImplementedError(url)
 
   return data.decode('utf-8')
 
diff --git a/nvchecker_source/aur.py b/nvchecker_source/aur.py
index 29c87da..6373cbe 100644
--- a/nvchecker_source/aur.py
+++ b/nvchecker_source/aur.py
@@ -67,7 +67,7 @@ class Worker(BaseWorker):
   ) -> None:
     task_by_name: Dict[str, Entry] = dict(self.tasks)
 
-    async with self.acquire_token():
+    async with self.task_sem:
       results = await _run_batch_impl(batch, aur_results)
       for name, version in results.items():
         r = RawResult(name, version, task_by_name[name])
diff --git a/nvchecker_source/none.py b/nvchecker_source/none.py
index 5427acd..20bf3fd 100644
--- a/nvchecker_source/none.py
+++ b/nvchecker_source/none.py
@@ -10,7 +10,7 @@ from nvchecker.api import (
 class Worker(BaseWorker):
   async def run(self) -> None:
     exc = GetVersionError('no source specified')
-    async with self.acquire_token():
+    async with self.task_sem:
       for name, conf in self.tasks:
         await self.result_q.put(
           RawResult(name, exc, conf))
diff --git a/tests/conftest.py b/tests/conftest.py
index b99e69d..741d989 100644
--- a/tests/conftest.py
+++ b/tests/conftest.py
@@ -18,7 +18,7 @@ use_keyfile = False
 async def run(
   entries: Entries, max_concurrency: int = 20,
 ) -> VersData:
-  token_q = core.token_queue(max_concurrency)
+  task_sem = asyncio.Semaphore(max_concurrency)
   result_q: asyncio.Queue[RawResult] = asyncio.Queue()
   keyfile = os.environ.get('KEYFILE')
   if use_keyfile and keyfile:
@@ -28,7 +28,7 @@ async def run(
     keymanager = core.KeyManager(None)
 
   futures = core.dispatch(
-    entries, token_q, result_q,
+    entries, task_sem, result_q,
     keymanager, 1,
   )