To provide a porous film stretchable with a high draw ratio by extrusion-molding without using a solvent a high molecular weight polyolefin resin hard to extrusion-mold, and a manufacturing method thereof.

The porous film is obtained by melt-kneading and extruding by a twin-screw extruder a resin composition comprising a polyolefin having a viscosity-average molecular weight of at least 300,000 with a non-reactive gas, cooling and solidifying the extruded polyolefin resin containing the non- reactive gas, removing the non-reactive gas from the cooled and solidified polyolefin resin, extruding the polyolefinic resin by a single-screw extruder and subjecting the extruded resin composition to a treatment for forming a porous structure. The manufacturing method of the porous film is also provided.