To obtain a polymer material having a high permeation factor, a high separation factor and satisfactory strength as a membrane, especially a material useful for a gas separating membrane fit to obtain oxygen enriched air.

This gas separating membrane is made of a polymer of at least one kind of propiolic ester represented by the formula HC≤C-CO2-R, a copolymer of a mixture of the propiolic ester with other copolymerizable monomer or a laminated body consisting of the (conpolymer and other material. In the formula, R is 1-18C alkyl, 4-12C cycloalkyl, aryl or a substituent obtd. by combining such substituents with each other and F atoms may be substd. for part or all of the H atoms in each of the substituents.