A polyolefin microporous membrane has a variation range of F25 value in a longitudinal direction of 1 MPa or less; a thickness of 3 μm or more and less than 7 μm; and a length of 1,000 m or more (wherein the F25 value is a value obtained by measuring a load value applied to a test specimen when the test specimen is stretched by 25% using a tensile tester; and dividing the load value by a value of a cross-sectional area of a test specimen).