PURPOSE: To prepare a semi-permeable membrane having excellent capacities in both of filtering efficiency and a water pervious speed, by forming the same from a solution of polysulfone resin in a specific solvent.

CONSTITUTION: Polysulfone resin is dissolved in 2-pyrolidone and the resulting solution is allowed to stand still and defoamed to obtain a uniform polymer solution. This solution is cast on a smooth glass plate at a speed of 4cm/sec by using a doctor blade having a gap of 150μm and, after the film is allowed to stand for one min, it is immersed in water with a temp. of 10°C along with the glass plate to gel the polymer. When the formed membrane peeled from the glass plate to be used in a flat membrane ultrafiltration apparatus (an available area 25cm2) and the permeation characteristics of an egg white albumin solution with a temp. of 25°C are measured under such a condition that a supply flow amount is 125ml/min and pressure is 0.5kg/cm2, the permeation speed thereof is 910ml/min-m2 and the solute permeation rate thereof is 1.32%. In addition, the coefficient of permeation of distilled water is 10.3m3/m2.day.kg/ cm2. By this method, the membrane having good capacities is obtained in good reproducibility without being affected by the properties of the base plate.