The present invention provides a composition for a CO2 gas separation membrane comprising: at least one compound selected from the group consisting of alkali metal carbonates, alkali metal bicarbonates, and alkali metal hydroxides; a crosslinked polymer in which a polymer having carboxyl groups has been crosslinked; and a non-crosslinked polymer obtained by polymerization of one or more monomers selected from the group consisting of vinyl acetate, acrylic acid, methacrylic acid, and derivatives thereof.