PURPOSE: To obtain a separation membrane having strong compatibility with an org. solvent, appropriate strength and flexibility, by using an epoxy resin based on an aliphatic compound with a carbon number of 6 or more having a reactive group as the material of said membrane.

CONSTITUTION: An aliphatic compound such as diglycidyl 7, 8-diphanyltetradecamethylene dicarboxylate having a curing catalyst mixed therein is cast on a polypropylene film and the obtained film is transferred onto a Teflon plate after preliminarily cured at a room temp. to be cured perfectly under heating to prepare a thin membrane made of an epoxy resin. In combining said membrane with a porous support, a solution of a mixture of the aliphatic compound and the curing catalyst in an inert solvent is coated on the surface of the support comprising cellulose acetate or glass and the formed coating is dried and cured to prepare a composite membrane.