PURPOSE: To obtain the title film having a large gas flux and excellent mechanical strengths, by infiltrating a crystallization promoting solvent into a molding of a crystalline polymer of a crystallinity ≤ a specified value and forming micropores in this molding.

CONSTITUTION: A crystalline polymer molding comprising a polyolefin, polyester, polyamide, vinyl, polyurethane or like polymer and having a form of film, tube, hollow fiber or the like is immersed in a solvent which promotes the crystallization of the crystalline polymer and can swell the molding to a degree of swelling ≥1.2 for 5minW2hr to infiltrate the solvent into the molding and to make the molding porous. This molding is optionally stretched at a draw ratio of 1.5W15 to obtain the title film composed of particulate aggregate of particles of a mean diameter of 0.1W10μ and a network structure formed of fibrils of an average width of 0.01W5μ and an average length of 0.5W50μ.