A microporous polyolefin membrane having large pore diameters and excellent air permeability, mechanical strength and compression resistance can be obtained by (a) stretching a gel molding comprising a polyolefin and a membrane-forming solvent at least uniaxially at a temperature from the crystal dispersion temperature of the polyolefin +15.degree. C. to the crystal dispersion temperature of the polyolefin +40.degree. C., removing the membrane-forming solvent, and then stretching again the resultant membrane to 1.1 to 2.5 fold at least uniaxially, or by (b) stretching the gel molding at least uniaxially, bringing the stretched film into contact with a hot solvent before and/or after removing the membrane-forming solvent, and then stretching again the resultant membrane to 1.1 to 2.5 fold at least uniaxially.