The invention provides a novel substituted polyacetylene copolymer having an outstandingly large oxygen permeation coefficient of 6.times.10.sup.-8 cm.sup.3 (STP).multidot.cm/cm.sup.2 .multidot.sec.multidot.cmHg or larger and a separation factor between oxygen and nitrogen of 2.0 or larger along with a high tensile strength of 200 kg/cm.sup.2 or larger in the form of a film and useful as a gas permeation element in the separation or enrichment of oxygen from or in air. The copolymer is obtained by the copolymerization of 1-trimethylsilyl-1-propyne and 1-(1',1',3',3'-tetramethyl-1',3'-disilabutyl)-1-propyne in a molar ratio of 30:70 to 95:5 and the copolymer should preferably have a number-average molecular weight of at least 1.times.10.sup.4.