We have developed a new type of asymmetric membranes having a homogeneous hyperthin skin layer, which was used as a polyimide synthesized by 2,2-bis(3,4-dicarboxyphenyl) hexafluoropropane dianhydride (6FDA) and 2,2-bis(4-amino phenyl) hexafluoro-propane (BAAF). The skin layer thicknesses of the 6FDA-BAAF polyimide asymmetric membranes were 40-60 nm, and the porosity was 10(-6)% when a defect size was assumed as 5 nm. The permselectivity of GFDA-BAAF polyimide asymmetric membranes after silicone coating had a of 40 for CO2/CH4 and a flux of 1.0 [Nm(3)/m(2)-h-atm] (=3.7 x 10(-4) [cm(3)(STP)/cm(2) s cmHg]) for CO2, alpha of 4.3 for O-2/N-2 and a flux of 2.0 x 10(-1) [Nm(3)/m(2)/h/atm] (=7.1 x 10(-5) [cm(3)(STP)/cm(2) s cmHg]) for O-2. These values were constant for large-scale manufacturing.