Disclosed herein is a multi-layer, microporous polyolefin membrane comprising a first porous layer of a polyethylene, and a second porous layer comprising a polyethylene, and polypropylene having a weight-average molecular weight of 6.times.10.sup.5 or more and a fraction having a molecular weight of 1.8.times.10.sup.6 or more being 10% or more by mass.