PURPOSE: To obtain a gas separation membrane excellent in permselectivity at a high permeation speed, by using a doctor knife of which the center line roughness is 0.05μm or less.

CONSTITUTION: A mixture consisting of polyvinyltriorganosilane and organopolysiloxane is used as a high-molecular membrane material. As polyvinylsilane, one with intrinsic viscosity of 1.5dl/g or more and a degree of dispersion of 2 or less is used and, as polysiloxane, one with a b.p. of 200°C or more and a number average MW of 1,000 or more is used. The mixing ratio of polyvinylsilane and polysiloxane is pref. 1:0.05W1. A membrane forming solution having this membrane material dissolved therein in an amount of 0.1W50wt% is formed into a membrane with a thickness of 500μm or more by using a doctor knife of which the center line average roughness is 0.05 or less. Further, it is necessary to use a cast plate with a contact angle of 40W70°.