PURPOSE: To prepare the titled microporous membrane having excellent permeation characteristics and selective permeability, and suitable especially as a substrate for a composite separation membrane, by forming a solution composed of a polysulfone resin, a solvent of the resin, a cationic metal salt, and an alcohol in the form of a film, and coagulating the film in a coagulation bath.

CONSTITUTION: A polysulfone resin solution is prepared by mixing (A) a polysulfone resin preferably a polymer containing ≥50mol% of the recurring unit of formula I or formula II with (B) a solvent which dissolves ≥15wt% of the polysulfone resin at ≤50°C, (C) a univalent or bivalent cationic metal salt and (D) a 2W15C monohydric alcohol. The concentrations of the resin, the salt and the alcohol in the solution are 10W35wt%, 0.3W7wt% and 10W55wt%, respectively, and the weight ratio of alcohol/polysulfone resin is ≤2.5. The solution is cast or spun to form a flat membrane or a hollow fiber, and the product is coagulated with a coagulation bath (especially a liquid composed essentially of water) to obtain a microporous membrane having an average surface pore diameter of ≤0.05μ.