diff --git a/tests/test_fenwicktree.py b/tests/test_fenwicktree.py
new file mode 100644
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--- /dev/null
+++ b/tests/test_fenwicktree.py
@@ -0,0 +1,249 @@
+import pytest
+from typing import List, Tuple
+
+from atcoder.fenwicktree import FenwickTree
+
+
+@pytest.fixture
+def fenwicktree() -> FenwickTree:
+    return FenwickTree(5)
+
+
+class TestFenwickTree:
+
+    def test_initial_status(self, fenwicktree: FenwickTree) -> None:
+        '''
+        An initialized fenwicktree object is expected to have all the elements
+        0.
+
+        GIVEN an initialized fenwicktree object
+        WHEN before executing fenwicktree.add(p, x)
+        THEN all the elements are 0
+        '''
+
+        assert fenwicktree.data == [0, 0, 0, 0, 0]
+
+        pair_of_positions = self._generate_pair_of_positions()
+
+        for left, right in pair_of_positions:
+            assert fenwicktree.sum(left, right) == 0
+
+    def _generate_pair_of_positions(self) -> List[Tuple[int, ...]]:
+        from itertools import combinations_with_replacement
+
+        return list(combinations_with_replacement(range(5), 2))
+
+    def test_add(self, fenwicktree: FenwickTree) -> None:
+        '''
+        fenwicktree.add(p, x) is expected to add x to the p-th of the number
+        sequence.
+
+        GIVEN an initialized fenwicktree object
+        WHEN add 1 to the first in the number sequence
+        THEN fenwicktree.data has [1, 1, 0, 1, 0]
+        '''
+
+        fenwicktree.add(0, 1)
+        assert fenwicktree.data == [1, 1, 0, 1, 0]
+
+    def test_add_multiple_times(self, fenwicktree: FenwickTree) -> None:
+        '''
+        fenwicktree.add(p, x) is expected to add x to the p-th of the number
+        sequence.
+
+        GIVEN an initialized fenwicktree object
+        WHEN add arbitrary value(s) at arbitrary positions in the sequence
+        THEN fenwicktree.data has a partial sum at any position in the sequence
+        '''
+
+        expected = [[1, 1, 0, 1, 0],
+                    [1, 3, 0, 3, 0],
+                    [1, 3, 3, 6, 0],
+                    [1, 3, 3, 10, 0],
+                    [1, 3, 3, 10, 5]
+                    ]
+
+        for i in range(5):
+            fenwicktree.add(i, i + 1)
+            assert fenwicktree.data == expected[i]
+
+    def test_add_when_negative_value_is_given(
+        self, fenwicktree: FenwickTree
+    ) -> None:
+        '''
+        fenwicktree.add(p, x) is expected to add x to the p-th of the number
+        sequence.
+
+        GIVEN an initialized fenwicktree object
+        WHEN add negative value at arbitrary positions in the sequence
+        THEN fenwicktree.data has a partial sum at any position in the sequence
+        '''
+
+        for i in range(5):
+            fenwicktree.add(i, i + 1)
+
+        assert fenwicktree.data == [1, 3, 3, 10, 5]
+
+        fenwicktree.add(0, -5)
+        assert fenwicktree.data == [-4, -2, 3, 5, 5]
+
+    def test_add_when_zero_is_given(self, fenwicktree: FenwickTree) -> None:
+        '''
+        fenwicktree.add(p, x) is expected to add x to the p-th of the number
+        sequence.
+
+        GIVEN an initialized fenwicktree object
+        WHEN add zero at arbitrary positions in the sequence
+        THEN fenwicktree.data has a partial sum at any position in the sequence
+        '''
+
+        for i in range(5):
+            fenwicktree.add(i, i + 1)
+
+        assert fenwicktree.data == [1, 3, 3, 10, 5]
+
+        fenwicktree.add(0, 0)
+        assert fenwicktree.data == [1, 3, 3, 10, 5]
+
+    def test_add_when_positive_floating_point_value_is_given(
+        self, fenwicktree: FenwickTree
+    ) -> None:
+        '''
+        fenwicktree.add(p, x) is expected to add x to the p-th of the number
+        sequence.
+
+        GIVEN an initialized fenwicktree object
+        WHEN add positive floating point value at arbitrary positions in the
+             sequence
+        THEN fenwicktree.data has a partial sum at any position in the sequence
+        '''
+
+        fenwicktree.add(0, 1.5)
+        assert fenwicktree.data == [1.5, 1.5, 0, 1.5, 0]
+
+    def test_add_when_negative_floating_point_value_is_given(
+        self, fenwicktree: FenwickTree
+    ) -> None:
+        '''
+        fenwicktree.add(p, x) is expected to add x to the p-th of the number
+        sequence.
+
+        GIVEN an initialized fenwicktree object
+        WHEN add negative floating point value at arbitrary positions in the
+             sequence
+        THEN fenwicktree.data has a partial sum at any position in the sequence
+        '''
+
+        fenwicktree.add(0, -1.5)
+        assert fenwicktree.data == [-1.5, -1.5, 0, -1.5, 0]
+
+    @pytest.mark.parametrize((
+        'left', 'right', 'expected'
+    ), [
+        (0, 5, 15),
+        (0, 4, 10),
+        (1, 4, 9),
+        (1, 3, 5),
+        (2, 2, 0),
+    ])
+    def test_sum(
+        self, fenwicktree: FenwickTree, left: int, right: int, expected: int
+    ) -> None:
+        '''
+        fenwicktree.sum(left, right) is expected to calculate the sum of the
+        interval [left, right)
+
+        GIVEN With arbitrary value(s) at arbitrary positions in the sequence
+              added to an initialized fenwicktree object
+        WHEN specify the interval [left, right)
+        THEN returns the sum of interval [left, right)
+        '''
+
+        for i in range(5):
+            fenwicktree.add(i, i + 1)
+
+        assert fenwicktree.sum(left, right) == expected
+
+    @pytest.mark.parametrize((
+        'left', 'right', 'expected'
+    ), [
+        (0, 5, 17),
+        (0, 4, 12),
+        (0, 3, 8),
+        (0, 2, 3),
+        (0, 1, 1),
+        (1, 4, 11),
+        (1, 3, 7),
+        (2, 2, 0),
+    ])
+    def test_sum_when_additional_element_is_given(
+        self, fenwicktree: FenwickTree, left: int, right: int, expected: int
+    ) -> None:
+        '''
+        fenwicktree.sum(left, right) is expected to calculate the sum of the
+        interval [left, right)
+
+        GIVEN With arbitrary value(s) at arbitrary positions in the sequence
+              added to an initialized fenwicktree object
+        WHEN specify the interval [left, right) after the additional element
+             is given
+        THEN returns the sum of interval [left, right)
+        '''
+
+        for i in range(5):
+            fenwicktree.add(i, i + 1)
+
+        assert fenwicktree.sum(0, 5) == 15
+
+        fenwicktree.add(2, 2)
+        assert fenwicktree.data == [1, 3, 5, 12, 5]
+        assert fenwicktree.sum(left, right) == expected
+
+    @pytest.mark.parametrize((
+        'p'
+    ), [
+        -2,
+        -1,
+        5,
+        6,
+    ])
+    def test_add_failed_if_invalid_input_is_given(
+        self, fenwicktree: FenwickTree, p: int
+    ) -> None:
+        '''
+        fenwicktree.add(p, x) is expected to be raised an AssertionError if an
+        invalid input is given.
+
+        GIVEN an initialized fenwicktree object
+        WHEN an out-of-range index is given
+        THEN raises an AssertionError
+        '''
+
+        with pytest.raises(AssertionError):
+            fenwicktree.add(p, None)
+
+    @pytest.mark.parametrize((
+        'left', 'right'
+    ), [
+        (-2, 5),
+        (-1, 5),
+        (0, 6),
+        (0, 7),
+        (3, 2),
+        (-1, 6),
+        (-1, 7),
+    ])
+    def test_sum_failed_if_invalid_input_is_given(
+        self, fenwicktree: FenwickTree, left: int, right: int
+    ) -> None:
+        '''
+        fenwicktree.sum(left, right) is expected to be raised an AssertionError
+        if an invalid input is given.
+
+        GIVEN an initialized fenwicktree object
+        WHEN an out-of-range index is given
+        THEN raises an AssertionError
+        '''
+
+        with pytest.raises(AssertionError):
+            fenwicktree.sum(left, right)