The present invention relates to a solid polymer electrolyte membrane having both of a higher proton conductivity and a smaller methanol permeability, which can be produced by conducting a graft polymerization of a fluororesin thin membrane irradiated with a radiation with a monofunctional monomer and again irradiating the resulting film with a radiation, followed by conducting a graft polymerization thereof with a polyfunctional monomer; and a high-performance fuel cell comprising the solid polymer electrolyte membrane, a fuel electrode and an air electrode, said solid polymer electrolyte membrane being disposed between the fuel electrode and the air electrode.