Abstract: A microporous membrane having a structure in which its pore size distribution curve obtained by mercury intrusion porosimetry has at least two peaks, which is produced by extruding a combination of a diluent or solvent and a polyolefin resin composition comprising (a) from about 74 to about 99% of a first polyethylene resin having a weight average molecular weight of from about 2.5×105 to about 5×105 and a molecular weight distribution of from about 5 to about 100, (b) from about 1 to about 5% of a second polyethylene resin having a weight average molecular weight of from about 5×105 to about 1×106 and a molecular weight distribution of from about 5 to about 100, and (c) from 0 to about 25% of a polypropylene resin having a weight average molecular weight of from about 3×105 to about 1.