Disclosed is a porous membrane consisting of a film-forming polymer characterized in that:(a) in at least one surface thereof, pores extending in a direction substantially perpendicular to the membrane surface are present at a surface pore density of 20 to 80%, said pores having an average pore diameter within the range of 0.01 to 100 .mu.m, a ratio of major to minor axis within the range of 1.0 to 2.0, and a coefficient of variation of pore diameter within the range of 0 to 50%; and(b) the overall porosity of said porous membrane is within the range of 20 to 90%.