The semipermeable membrane contains a polyarylate resin and a polysulfone resin as a major membrane material and has a fibrilliform structure with a high mechanical strength and has a water permeability of 1,500 ml/m.sup.2.hr.mmHg or lower as well as an inhibitory rate against bovine serum albumin of 80% or higher and against cytochrome C of 40% or lower. The membrane is prepared by discharging a stock solution containing the polyarylate resin and the polysulfone resin in the total amount of 10% to 25% by weight and a core solution of an organic solvent containing water in the amount of 25% to 80% by volume into a coagulating solution.