A microporous polyolefin membrane comprising a polyethylene resin, and polypropylene having a weight-average molecular weight of 6.times.10.sup.5 or more and a heat of fusion of 90 J/g or more (measured by a differential scanning calorimeter), a fraction having a molecular weight of 5.times.10.sup.4 or less being 5% or less by mass of the polypropylene.