Patent ID: 7479243

Claim:
A method of producing a microporous polyolefin membrane, comprising the steps of extruding a solution composed of 10 to 50 weight % of (A) a polyolefin having a weight-average molecular weight of 5×10 5 or more or (B) a composition containing this polyolefin and 50 to 90 weight % of a solvent, into a gel-like formed article and removing the solvent therefrom, wherein a treatment step with a hot solvent is performed on the gel-like formed article, and wherein the average pore size (a) in the area from the surface to a depth of d/16 (d: thickness of said membrane) is 0.05 to 50 μm, and the average pore size (b) in the other area is 0.01 to 30 μm, where the relationship (b)<(a) holds.