A method is presented for producing polyolefin microporous membranes which are superior in thermal stability and are particularly useful as a separator for a lithium ion battery. A process including a first step of melting polyolefin resin and mixing together at least melted polyolefin resin, organosiloxane particles including a polysiloxane cross-linked structure and having a spherical or golfball shape with an average particle diameter of 0.01-10 μm and a plasticizer to obtain a melted mixture, a second step of molding this mixture and biaxially stretching molded product to obtain a stretched film and a third step of extracting and removing the plasticizer from the stretched film is carried out, if a membrane having a single film layer is to be produced, to obtain this single film layer and, if a membrane having two or more laminated film layer is to be produced, to obtain the film layers on both outsides.