A polyolefin microporous membrane having an air permeability normalized to a thickness of 16 μm of from 100 to 220 sec/100 cc measured in accordance with JIS P8117, a pin puncture strength normalized to a thickness of 16 μm of not less than 5.39 N (550 gf) and measured according to the description, wherein the polyolefin contains not less than 30 wt.% and not more than 80 wt.%, based on 100 wt.% of the total amount of the polyolefin resin, of an ultra-high molecular weight polyethylene having a weight average molecular weight of not less than 1.0 x 10 6 and less than 2.0 x 10 6 , and high-density polyethylene, and obtainable by the following steps: (a) preparing a polyolefin solution by melt-kneading (i) a polyolefin resin containing an ultra-high molecular weight polyethylene having a weight average molecular weight of not less than 1.0 x 10 6 and less than 2.0 x 10 6 and a high-density polyethylene; and (ii) a membrane-forming solvent; and wherein the polyolefin resin constitutes from 10 to 50 wt.% based on 100 wt.% of the total amount of the polyolefin and the membrane-forming solvent; (b) molding a gel-like sheet by extruding the molten mixture obtained in step (a) from an extruder to form an extrudate and then cooling the extrudate at a rate of not less than 250°C/min until a temperature not higher than the crystallization completion temperature is reached on both the front and back of the gel-like sheet; and wherein the crystallization half time t 1/2 of the gel-like sheet, measured according to the description, is from 10 to 35 minutes when subjected to isothermal crystallization at 117 °C; and wherein the crystallization completion temperature is the extrapolated crystallization completion temperature measured in accordance with JIS K721 ; (c) stretching the gel-like sheet obtained in step (b) in a longitudinal direction (machine direction); (d) stretching the sheet obtained in step (c) in a transverse direction (direction orthogonal to the machine direction) sequentially; (e) extracting a membrane-forming solvent from the stretched film obtained in step (d); and (f) drying the microporous membrane obtained in step (e). (a) preparing a polyolefin solution by melt-kneading (i) a polyolefin resin containing an ultra-high molecular weight polyethylene having a weight average molecular weight of not less than 1.0 x 10 6 and less than 2.0 x 10 6 and a high-density polyethylene; and (ii) a membrane-forming solvent; and wherein the polyolefin resin constitutes from 10 to 50 wt.% based on 100 wt.% of the total amount of the polyolefin and the membrane-forming solvent; (b) molding a gel-like sheet by extruding the molten mixture obtained in step (a) from an extruder to form an extrudate and then cooling the extrudate at a rate of not less than 250°C/min until a temperature not higher than the crystallization completion temperature is reached on both the front and back of the gel-like sheet; and wherein the crystallization half time t 1/2 of the gel-like sheet, measured according to the description, is from 10 to 35 minutes when subjected to isothermal crystallization at 117 °C; and wherein the crystallization completion temperature is the extrapolated crystallization completion temperature measured in accordance with JIS K721 ; (c) stretching the gel-like sheet obtained in step (b) in a longitudinal direction (machine direction); (d) stretching the sheet obtained in step (c) in a transverse direction (direction orthogonal to the machine direction) sequentially; (e) extracting a membrane-forming solvent from the stretched film obtained in step (d); and (f) drying the microporous membrane obtained in step (e). <EOS>