Certain polymeric compounds such as amide-imide polymers, imide polymers, sulfone polymers, fluorocarbon polymers and olefin polymers are known as materials for constituting porous films. Porous films are produced from these materials, for example, by a phase conversion technique in which a mixture containing the polymeric compound is cast as a film and the film is brought to a solidifying liquid. The resulting film produced by the phase conversion technique using the polymeric compound as a material, however, has a skin layer (compact layer) on its surface and contains substantially no hole area (opening) or contains some openings with a low rate of hole area. Porous films using a polyimide, a kind of imide polymers, as a material and production thereof, for example, are disclosed in Japanese Unexamined Patent Application Publication No. 2001-67643, No. 2001-145826 and No. 2000-319442. These films, however, must be produced using a control member for a solvent replacement rate, thereby require complicated production processes and have insufficient rates of hole area and permeability.
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