PURPOSE: To obtain a gas separation membrane excellent in oxygen permeability, by using a polyurethane/silicone copolymer prepared by introducing a polysiloxane structure into the main chain of polyurethane.

CONSTITUTION: A polyurethane/silicone copolymer obtained by heating and reacting diisocyanate and polysiloxane type diol in an org. solvent or a ternary copolymer obtained by further using diol as a third component is dissolved in a film forming solvent to be cast and applied onto an appropriate support substrate and the solvent is evaporated from the formed film by heat treatment to obtain a homogenous membrane with a thickness of 0.05W30μm. When sputter etching treatment is applied to the surface of this membrane, coefficient of gas permeation and coefficient of gas separation are further enhanced.