PURPOSE: To obtain a gas separation membrane having good gas permeability and good gas separability by forming poly-di-substituted-acetylene having a perfluoroalkyl group in the side chain thereof into a film.

CONSTITUTION: Poly di-substituted acetylene represented by formula II (wherein m is an integer of 0 or more and A is a 1-20C alkyl group or a silaalkyl group) is dissolved in a solvent such as carbon tetrachloride and brominated with N-bromosuccinic imide in the presence of a catalyst such as benzoyl peroxide to be reacted with sodium perfluoroalkoxide. As a result, a di-substituted acetylene polymer represented by formula I and formula II (wherein n is an integer of 1 or more, X is -Rf or -Y-Rf. Herein, Rf is a perfluoroalkyl group,and Y is an oxygen atom or a divalent org. group), having an m/n ratio of 0/100-90/5 and an MW of at least 10000 or more is obtained. This polymer is dissolved in a solvent such as chloroform and formed into an ultrathin film by a water casting method.