PURPOSE: To improve desalting performance and oxidation resistance in polyamide having aliphat. primary amide bonds by selecting acid components adequately.

CONSTITUTION: Said composite membrane is obtained by causing crosslinking reaction of aliphat. diamines such as ethylenediamine and cyclohexanediamine with arom. polyacid halide on a porous membrane of polysulfone, etc. Said aliphat. diamines may be used alone and it is possible to control the rate of water permeation and the rate of desalting of the composite membrane by using other polyamine compds., for example, arom. polyamines, aliphat. secondary diamines, and cyclic aliphat. diamines in combination. Said composite membrane is provided with incredibly high oxidation resistance in addition to basic reverse osmotic performance, heat resistance, resistance to consolidation or to changes of pH.