PURPOSE: To obtain a surface modified film having a high filtering speed by irradiating a porous membrane made of polysulfone with ultraviolet rays at room temp. under atmospheric pressure.

CONSTITUTION: A porous membrane made of polysulfone is irradiated with ultraviolet rays having proper intensity at room temp. under atmospheric pressure while an irradiation distance and an irradiation time are properly set. For example, the proper intensity of ultraviolet rays is about 600W, the proper irradiation distance is about 25cm and the proper irradiation time is 1-20min. The obtained modified membrane is extremely low in the compatibility with protein and solves the sudden lowering of a filtering speed due to the adsorption of protein and keeps a high filtering speed. Therefore, a filtering time in a wide range of filtering operation such as the filtering of protein is shortened to a large extent.