PURPOSE: To enhance a gas permeability and a separating power of a membrane by preparing a layer consisting of a layer which essentially comprises a high polymer having at least a gas permeability and a layer comprising a high polymer of a fumaric acid diester as a major ingredient, on a porous carrier.

CONSTITUTION: A layer essentially comprising a high polymer having the gas permeability such as a crosslinked polysiloxane, e.g., a polydimethylsiloxane, etc., and a layer essentially comprising a polymer of the fumaric acid diester such as a poly(diisopropyl fumarate) is prepared on a porous carrier, e.g., a polysulfone, the pore size of which is ranged within 10W10,000 by means of a water surface expansion method, etc., to obtain a composite membrane for gas separation. The membrane thus obtained is excellent in the O2 permeating rate and the separation of N2.