Vinylidene fluoride porous membrane and method of preparing the same

A vinylidene fluoride porous membrane which comprises a vinylidene fluoride resin, said membrane having pores which satisfy the following numerical expression (A):

• 4.0<P.sub.1 /P.sub.2 .ltoreq.10.00 (A)
• in which P.sub.1 is an average pore diameter in a surface which has a larger average pore diameter, and P.sub.2 is an average pore diameter in the other surface of the membrane;
• wherein the inherent viscosity of the vinylidene fluoride resin which constitutes the porous membrane is from 1.3 to 15.0 when measured on a sample of the porous membrane, and the following numerical expression (B) is satisfied:
• (Tb.times.Eb/(100-.phi.).times.100.gtoreq.1,000 (B)
• wherein .phi. is a porosity in % by volume, Tb is a tensile strength at break in MPa, and Eb is % elongation at the point of break.