A microporous film, characterized in that it is prepared by a method comprising melting and kneading a polyolefin resin, inorganic particles and a plasticizer, forming the resultant mixture into a sheet, subjecting the rejecting the resultant sheet to a biaxial stretching treatment of a surface magnification of not less than 20 and less than 100 times, and then extracting the plasticizer, it has an inorganic particle content of 20 to 60 mass %, and it exhibits a piercing strength of 3.0 N/20 μm or more.