diff --git a/tests/test__bit.py b/tests/test__bit.py
new file mode 100644
index 0000000..7e50228
--- /dev/null
+++ b/tests/test__bit.py
@@ -0,0 +1,54 @@
+import pytest
+
+from atcoder._bit import _ceil_pow2, _bsf
+
+
+class TestInternalBit:
+
+    @pytest.mark.parametrize((
+        'n', 'expected'
+        ), [
+        (0, 0),
+        (1, 0),
+        (2, 1),
+        (3, 2),
+        (4, 2),
+        (5, 3),
+        (6, 3),
+        (7, 3),
+        (8, 3),
+        (9, 4),
+        ((1 << 30) - 1, 30),
+        (1 << 30, 30),
+        ((1 << 30) + 1, 31),
+        ((1 << 100) - 1, 100),
+        (1 << 100, 100),
+        ((1 << 100) + 1, 101),
+    ])
+    def test_ceil_pow2(self, n: int, expected: int) -> None:
+        assert _ceil_pow2(n) == expected
+
+    @pytest.mark.parametrize((
+        'n', 'expected'
+        ), [
+        (1, 0),
+        (2, 1),
+        (3, 0),
+        (4, 2),
+        (5, 0),
+        (6, 1),
+        (7, 0),
+        (8, 3),
+        (9, 0),
+        ((1 << 30) - 1, 0),
+        (1 << 30, 30),
+        ((1 << 30) + 1, 0),
+        ((1 << 31) - 1, 0),
+        (1 << 31, 31),
+        ((1 << 31) + 1, 0),
+        ((1 << 100) - 1, 0),
+        (1 << 100, 100),
+        ((1 << 100) + 1, 0),
+    ])
+    def test_bsf(self, n: int, expected: int) -> None:
+        assert _bsf(n) == expected