Abstract: A microporous membrane made of polyolefins which comprises polyethylene (PEA) 8-60 wt. % of which is accounted for by components having a molecular weight of 10,000 or lower and in which the ratio of the weight-average molecular weight (Mw) to the number-average molecular weight (Mn), Mw/Mn, is 11-100 and the viscosity-average molecular weight (Mv) is 100,000-1,000,000 and polypropylene, and which has a content of components having a molecular weight of 10,000 or lower of 8-60 wt. %, a porosity of 20-95%, and a degree of thermal shrinkage at 100° C. of 10% or lower.