PURPOSE: To obtain an oxygen enriching membrane excellent in oxygen permeability and separation efficiency, by forming a plasma polymerization membrane of bis(trimethylsilyl)acetamide to the surface of a porous membrane material.

CONSTITUTION: A porous membrane material comprising polypropylene, polycarbonate, polysulfone, polyvinylidene fluoride or cellulose acetate is used in a flat membrane or hollow yarn shape having a thickness suitable to hold sufficient strength as a support. A bis(trimethylsilyl)acetamide monomer alone or a mixture of said monomer and ammonia is polymerized on said membrane material under pressure of 0.02W0.1Torr in a plasma polymerization apparatus to form a plasma polymerization membrane. The oxygen transmitting speed of the gas separation membrane under such a condition that monomer pressure is 0.08Torr, output is 30W and a time is 30min is 2.26×10-3.cm3.cm/ cm2.sec.cm.Hg and the nitrogen separation factor thereof is 3.4.