PURPOSE: To impart a hydrophobic porous membrane with hydrophilic property having excellent durability, by keeping a crosslinkable copolymer composed of a hydrophilic vinyl monomer and a crosslinkable vinyl monomer on the surface of pores of said membrane and crosslinking the copolymer.

CONSTITUTION: (A) A hydrophilic vinyl monomer such as (meth)acrylic acid, hydroxymethyl methacrylate etc., is made to react with (B) a crosslinkable vinyl monomer (preferably diacetone acrylamide) at a weight ratio (A/B) of preferably 99.5/0.5W50/50 under a mild condition to prevent the reaction of the crosslinkable functional group of the component B. The obtained copolymer having a molecular weight of preferably 10,000W500,000 is dissolved in e.g. acetone and is held on the surface of at least a part of pores of a hydrophobic porous membrane e.g. by immersing the membrane in the solution. The copolymer is heated and crosslinked keeping the above state to effect the hydrophilization of said porous membrane.