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

The porous polyolefin film is obtained by tandem extrusion comprising melt-kneading 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, removing a part of the non-reactive gas before the resin composition solidifies, subsequently feeding the resin composition into a single-screw extruder, removing the whole quantity of the non-reactive gas in the single-screw extruder and extruding a dense resin composition and subjecting the thus-obtained dense resin composition to a treatment for forming a porous structure. The manufacturing method of the porous polyolefin film is also provided.