A method for producing a microporous membrane, the method comprising: 1) melt-kneading a primary material having a molecular weight (Mw) of less than 1.0×10 6 measured by a high temperature GPC apparatus as outlined in the description, which comprises a high density polyethylene resin having an average particle diameter of 105μm or more, a secondary material comprising an ultrahigh molecular weight polyethylene resin having a molecular weight of 1.0×10 6 or more and an average particle diameter of 90 μm or more, and a plasticizer which has no exothermic reaction at 100 to 120°C, wherein the ratio of the average particle diameters of the high density polyethylene resin and ultrahigh molecular weight polyethylene resin is 0.35 or more and 1.0 or less, the proportion of ultrahigh molecular weight polyethylene resin in the polyethylene resins is 6 wt% or more, and the concentration of the polyethylene resins in the mixture with plasticizer is from 10 to 50 wt%, and wherein two or more additions of the plasticizer are made with the ratio of addition on the upstream side being from 50 to 95 wt%; 2) extruding the molten mixture obtained in the step 1) through a spinneret to mold into a sheet shape; 3) stretching the sheet obtained in the step 2) by a sequential stretching method including a roll system or a tenter system; and 4) extracting the plasticizer from the stretched film obtained in the step 3) to produce a polyolefin microporous membrane, wherein, when an endothermic quantity of a mixture of the primary material and the plasticizer and an endothermic quantity of a mixture of the secondary material and the plasticizer measured by DSC at a heating rate of 10°C / min are denoted as Q1 and Q2, respectively, a ratio of the endothermic quantity Q2 to the endothermic quantity Q1, (endothermic quantity Q2/endothermic quantity Q1), is 1 or more over a temperature range from 110 to 118°C. 1) melt-kneading a primary material having a molecular weight (Mw) of less than 1.0×10 6 measured by a high temperature GPC apparatus as outlined in the description, which comprises a high density polyethylene resin having an average particle diameter of 105μm or more, a secondary material comprising an ultrahigh molecular weight polyethylene resin having a molecular weight of 1.0×10 6 or more and an average particle diameter of 90 μm or more, and a plasticizer which has no exothermic reaction at 100 to 120°C, wherein the ratio of the average particle diameters of the high density polyethylene resin and ultrahigh molecular weight polyethylene resin is 0.35 or more and 1.0 or less, the proportion of ultrahigh molecular weight polyethylene resin in the polyethylene resins is 6 wt% or more, and the concentration of the polyethylene resins in the mixture with plasticizer is from 10 to 50 wt%, and wherein two or more additions of the plasticizer are made with the ratio of addition on the upstream side being from 50 to 95 wt%; 2) extruding the molten mixture obtained in the step 1) through a spinneret to mold into a sheet shape; 3) stretching the sheet obtained in the step 2) by a sequential stretching method including a roll system or a tenter system; and 4) extracting the plasticizer from the stretched film obtained in the step 3) to produce a polyolefin microporous membrane, <EOS>