Abstract: A polyolefin microporous membrane having a film thickness of 1 ?m to 30 ?m; an air permeability of 500 sec/100 cm3 or less; and a withstand voltage reduction rate of 1.0% or greater and 17.0% or less due to pressing under conditions of a temperature of 60° C., a pressure of 3.4 MPa, and a compression time of 1 sec or a withstand voltage reduction rate of 1.0% or greater and 28.0% or less due to pressing under conditions of a temperature of 70° C., a pressure of 8 MPa, and a compression time of 3 min, or having a polyethylene crystal long period before compression of 35.0 nm or more, measured by a small-angle X-ray scattering (SAXS) method, and a diffraction peak-to-peak distance of 2.410° or more and 2.600° or less derived from a (110) plane and a (200) plane of a polyethylene crystal before compression is provided.

Abstract: A polyolefin porous film is provided which has a film thickness of 1-30 ?m and an air permeability of at most 500 sec/100 cm3, and which has a 30% or higher porosity after compression, measured in a compression test under the conditions in which the temperature is 70° C., the pressure is 8 MPa and the compression time is 3 minutes.