Abstract: Disclosed is a membrane for the separation of a water-alcohol mixed liquid by the pervaporation method, which is composed of a chitosan having a molecular weight of 80,000 to 150,000 and a deacetylation degree adjusted to 80 to 95%. This separating membrane has an excellent selective permeability to water in the separation of a water-alcohol mixed liquid, and the mechanical strength of this separating membrane is very high. This separating membrane can be prepared by a process comprising dissolving a chitosan having a molecular weight of 80,000 to 150,000 and a deacetylation degree adjusted to 80 to 95% in an acidic aqueous solution to form a dope having a chitosan concentration of 9 to 12% by weight, shaping the dope into a membrane and immersing the membrane in an alkaline solution.