We describe the syntheses of novel sulfonated graft copolyimides and investigate their proton conductivity and gas permeability properties. The graft copolyimide membrane showed significantly higher proton conductivity exceeding that of Nation, and the proton conductivity at 90 degrees C and 98% RH was 1.2 S/cm, more than 7 times that of Nation. Furthermore, the oxygen permeability of the membrane was approximately one-eighth the value of Nation and had a very excellent gas barrier property. The selectivity ratio calculated from the proton and oxygen transports of the graft copolyimide membrane was 66 times that determined in Nation. We first demonstrated that the graft copolymer membrane could realize a high proton conductivity and low gas permeability of the fuel. Consequently, this material proved to be promising as a proton exchange membrane and may have potential applications for use in fuel cells.