PURPOSE:To prepare a permselective membrane of superb membrane preparing properties, reverse osmosis properties and chlorine resistance by reacting aromatic polycarboxylic acid component with mixed diamine component consisting of piperazine and 4,4'-diaminediphenyl sulfone. CONSTITUTION:Piperazine diamine, 4,4'-diaminodiphenyl sulfone and its derivative are mixed to form a diamine component. Mixture proportion is 5/95-65/35 mol ratio. As aromatic polycarboxylic acid component, terephthalic acid and isophthalic acid are used and reacted with diamine component to prepare copolyamide. Solution copolymerization or interfacial copolymerization is carried out. Polyamide thus prepared is dissolved in an adequate organic solvent to prepare a membrane. To adjust discharge of pore diameter of the membrane, micropore forming agents such as inorganic salt, glycerin and the like may be added to copolyamide solution. The thickness of membrane is adjusted as 0.05-1mu.