PURPOSE: To obtain a modified sulfonated polyolefin porous membrane capable of permitting water to pass from a dry state by bringing the inner surfaces of the pores of a porous membrane into contact with a solution containing a water-soluble polymer and subsequently drying the porous membrane.

CONSTITUTION: The inner surfaces of the pores of a porous membrane are brought into contact with an aqueous solution containing a water-soluble polymer (prepared by dissolving either one of polyvinyl pyrrolidone, polyvinyl alcohol, polyethylene glycol and dextran or a mixture thereof in a solvent having high affinity with respect to polyolefin and dissolving those water-soluble polymers such as ethanol or methanol) and subsequently dried. Whereupon, the hydrophilic water-soluble polymer is laminated to the inner surfaces of the pores of the membrane becoming hydrophilic by sulfonation and the hydrophilicity of the inner surfaces of the pores is markedly increased and the membrane capable of permitting water to pass from a dry state is obtained. Therefore, hydrophilicity imparting treatment at the time of the use of the membrane is unnecessary and it is unnecessary to enclose water in a module in order to prevent drying.