A polyolefin microporous membrane includes a polyethylene composition, and 0.5% or more but less than 5% by mass of polypropylene, wherein the polyethylene composition includes polyethylene having a mass average molecular weight of 2.5×105 to 5×105 and ultrahigh molecular weight polyethylene having a mass average molecular weight of 1×106 to 3×106 in an amount of more than 5% by mass and not more than 50% by mass based on 100% by mass of polyethylene composition and has a molecular weight distribution (Mw/Mn) of 5.0 to 300, and the polyolefin microporous membrane has an injection of electrolyte of 20 seconds or less and a uniform polypropylene distribution in at least one plane perpendicular to the thickness direction.