Process of making vinylidene fluoride porous membrane

A method of preparing a vinylidene fluoride porous membrane having 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, measured in .mu.m, in a surface which has a larger average pore diameter, and P.sub.2 is an average pore diameter, measured in .mu.m, in the other surface of the membrane, the method comprising the steps of:
• adding 30 to 250 parts by weight of an aliphatic polyester, as a plasticizer, and 1 to 50 parts by weight of a good solvent to 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., from 1.4 to 15.0 dl/g, to form a composition,
• extruding the so-formed composition to form a molten film,
• cooling one surface of the extruded 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.