PURPOSE: To obtain a membrane having steam permeability independent of the ratio of steam in a mixed gas, by use of a polymer comprised of a monomer contg. an ionic group and a vinyl alcohol monomer as a steam permselective membrane.

CONSTITUTION: A polymer comprised of a monomer having an ionic radical such as quaternary ammonium salts, amino groups, carboxyl groups, sulfonyl groups, phosphonyl groups, etc., and a vinyl alcohol monomer is used as a material for a steam permselective membrane. By using the material, a membrane having stable steam permeability independent of the ratio of steam in a mixed gas is obtained, in other words, the membrane has separation function not lowering even if the steam content is low. The membrane can be utilized to dehumidify or dry a gas such as nitrogen for general use, hydrogen chloride corrosive gas, natural gas, fermentation gas, etc.