A polyimide resin semipermeable membrane has a continuous structure of a homogeneous skin layer part having an average thickness of 5-1000 nm and a porous layer that is made of the same materials and has the same thickness as the skin layer. The porous layer is formed by the void structure having the average hole diameter of less than 3 .mu.m so that a gas separation membrane can be provided, having high permeability and satisfying cost efficiency requirements can be provided. The above-mentioned membrane is formed by the following steps: a solution is made by adding diethylene glycol dimethyl ether into polyimide; the obtained solution is left for deaeration and adjusted; the film-forming solution is cast on a polyester nonwoven cloth, which serves as a supporting layer, with an applicator; the solution is soaked in water as a solidifying solution for 5 minutes and in a water. Thus semipermeable membrane comprising the homogeneous skin layer part and the porous layer. Crosslinking silicone resin solution may be formed on the surface of the above-mentioned semipermeable membrane to form the overcoat.