PURPOSE: To obtain the title membrane enhanced in the transmission stability of transmitted gas and improved in volume efficiency by forming a membrane composed of the crosslinked composition of polyorganosiloxane and poly(1- trimethylsilyl-1-propine) to the surface of a porous support.

CONSTITUTION: A gas separation membrane is formed to the surface of a porous support composed of cellulose acetate or polypropylene and formed from the crosslinked composition consisting of polyorganosiloxane represented by general formula (-SiRR'-O-)n (wherein R is a vinyl group and R' is a lower alkyl group) and poly(1-trimethylsilyl-1-propine). Crosslinking reaction is performed in a nitrogen gas atmosphere in such a state that a radical polymerization initiator such as azobis-isobutylonitrile is added to a solvent such as aromatic hydrocarbon, for example, benzene or toluene. This crosslinked composition is formed into a membrane to be cast on the porous support and dried naturally. By this method, a gas permeable membrane excellent in the transmission stability of transmitted gas.