PURPOSE: To obtain a porous membrane with high separation efficiency and strength by continuously or laminarly forming a porous layer having a large number of direct pore-type pores and a microporous porous layer.

CONSTITUTION: In a porous polymer membrane used for a precision filtration membrane and an ultrafilter, a polymer membrane is made into a microporous membrane by mechanical stretching or using the difference of dissolution in a solvent, and then a polymer-dissolving coating solution is applied on the membrane or a polymer membrane is laminated on the membrane. After that, ion is radiated to the membrane to form radiation damage in the dense layer and then chemical etching is carried out. Thus, since the membrane has a direct- type porous layer 1 with uniform diameter of pores, the separation efficiency is high and since a three dimensional net-like microporous layer 2 works as a support and a reinforcing body, the membrane can have high strength.