PURPOSE: To enhance water permeability without loss of heat resistance, chemical resistance and mechanical strength by carrying out oxidation treatment of the surface of micro-porous membrane comprising polyphenylene sulfide as main component.

CONSTITUTION: Polymer containing more than 90mol% of p-phenylene sulfide is used as polyohenylene sulfide. Less than 35% of other polymer can be blended with said polymer. Said polyphenylene sulfide is molded in a shape suitable for use. Next, the surface of membrane is treated to turn hydrophilic by oxidizing sulfur of p-phenylene sulfide, main component of polyphenylene sulfide. For oxidizing treatment, hydrogen peroxide, peracetic acid, nitrogen dioxide, permanganate, periodate, chromate and the like are used for oxidization treatment.