PURPOSE: To obtain a semipermeable membrane having good filtering efficiency and a high water permeation speed, in a semipermeable membrane for separation based on a hydrophobic high-molecular substance having an asymmetric structure consisting of a dense layer and a porous layer, by constituting only the dense layer of said semipermeable membrane from a mixture consisting of a hydrophobic high-molecular substance and a hydrophilic high-molecular substance.

CONSTITUTION: As a hydrophobic polymer, an aromatic polysulfone represented by formulae I WIII is used and, as a hydrophilic polymer, polyvinyl pyrrolidone is used and both of them are dissolved in a polar org. solvent to prepare a dope which is, in turn, cast onto a sheet like or tubular support while the coated support is subsequently immersed in a coagulant bath to prepare an asymmetric separation semipermeable membrane by sol-gel phase conversion. By this method, an asymmetric structure, such that the hydrophilic polymer is present in the dense layer side of the membrane but no hydrophilic polymer is present in the porous layer side of the membrane back surface, is obtained and the water permeability of the formed membrane is improved while the heat resistance and chemical resistance of the aromatic polysulfone are kept.