A polyolefin microporous membrane is suitable to provide thereon a porous layer having little variation in thickness, which has a fluctuation range of F25 value in the length direction of 1 MPa or less, and which has a length of 1,000 m or more (wherein the F25 value refers to 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 the value of the cross-sectional area of the test specimen).