PURPOSE: To obtain a semipermeable membrane of superior filtration characteristics with good reproducibility by controlling the state in which membrane materials are gelled and solidified by the effect of a solidifying bath from a casting soln. using a high b.p. solvent in such a way that said state enters a specific range.

CONSTITUTION: The aliphat. org. acids expressed by the formulaI(where R1 is hydrogen or a carobxylic group in the case of n=0, hydrogen, methyl group, etc., in the case of n≥1, R2 is hydrogen, hydroxyl group, etc., R3 is hydrogen, methyl group, etc.) or their ester or a material expressed by the formula II (where R4WR8 are hydrogen, hydroxyl groups, etc., R9 is carboxylic group, etc. The third group is carboxylic acid amide, alkyl phosphate, etc.) are used as the material which alone does not dissolve polysulfone resin. The concn. of the polysulfone resin in the casting soln. is 5W40wt%, and the thickness is 50W 1,000μ. A glass plate, a metallic plate, etc. are used for the base plate for casting.