Disclosed are a gas separation membrane and a gas separation method in which at least one species of organic vapor is separated and recovered from an organic vapor mixture using the gas separation membrane. The gas separation membrane is made of an aromatic polyimide composed of a tetracarboxylic acid component consisting of an aromatic ring-containing tetracarboxylic acid and a diamine component comprising 10 to 90 mol % of a combination of (B1) 3,4′-diaminodiphenyl ether and (B2) 4,4′-diaminodiphenyl ether at a B1 to B2 molar ratio, B1/B2, ranging from 10/1 to 1/10, and 10 to 90 mol % of other aromatic diamine.