Abstract: A method of making a polyvinylidene fluoride porous membrane having an asymmetric structure, excellent mechanical strength and a narrow range of pore diameter distribution comprises: blending (i) 100 parts by weight of a vinylidene fluoride resin having an inherent viscosity, measured in dimethylformamide at a concentration of 0.4 g/dl and a temperature of 30.degree. C., of 1.4 to 15.0, (ii) 30 to 250 parts by weight of an aliphatic polyester plasticizer and, optionally and preferably, (iii) 1 to 50 parts by weight of a good solvent for vinylidene fluoride resin, to form a composition; extruding the so-formed composition to form a molten film; cooling one surface of the molten film by contact with a chill roll having a temperature of not more than 150.degree. C. and air-cooling the other surface to make a solid film; and extracting the plasticizer from the solid film.

Abstract: A polyvinylidene fluoride porous membrane has an asymmetrical structure, excellent mechanical strength such as tensile strength at break and elongation at break, and a desirable narrow range of pore diameter distribution. A method of preparing the porous membrane is also disclosed. The membrane comprises a vinylidene fluoride resin having an inherent viscosity from 1.3 to 15.0 (dl/g), and the membrane comprises pores which satisfy the following numerical expression (A):4.0<P.sub.1 /P.sub.2 .ltoreq.10.00 (A)wherein P.sub.1 is an average pore diameter (.mu.m) in a surface which has a larger average pore diameter, and P.sub.2 is an average pore diameter (.mu.m) in the other surface of the membrane.