A polyolefin microporous membrane has excellent strength, permeability and heat resistance, which is obtained by using UHMwPE and employing a sequential stretching system, and a production method of the microporous membrane. In producing a microporous membrane by using a primary material A having a molecular weight (Mw) of less than 1.0×106, a secondary material B having a molecular weight of 1.0×106 or more, and a plasticizer, when the endothermic quantity of a mixture of the primary material and the plasticizer and the endothermic quantity of a mixture of the secondary material and the plasticizer are denoted as Q1 and Q2, respectively, respective resins are designed such that the ratio of endothermic quantity Q2 to endothermic quantity Q1 (endothermic quantity Q2/endothermic quantity Q1) becomes 1 or more over a temperature range of 110 to 118° C.