TECHNOLOGY FOCUS - POLYMERS - Preferred Film: One surface of the electrolyte film comprises a composition obtained by graft polymerizing a hydrocarbon-type compound, and another surface of the electrolyte film comprises a composition (C) obtained by graft polymerizing a fluorine-containing hydrocarbon-type compound. The surface of the film at the air electrode side is comprised by the composition (C). Preferred Resin: The fluorine resin is tetrafluoro ethylene resin, tetrafluoro ethylene-hexafluoro propylene copolymer resin, tetrafluoro ethylene-perfluoro-alkyl vinyl ether copolymer resin, tetrafluoro ethylene-ethylene copolymer resin and/or vinylidene fluoride resin. The fluorine resin is obtained by irradiating with a radiation by an absorption dose of 5 kGy or more at a temperature beyond its melting point, and then irradiating with an electron beam at 20-40degreesC. EXAMPLE - A polytetrafluoro ethylene (PTFE) film was irradiated with an electron beam at acceleration voltage of 130 kV and exposure of 30 kGy. The irradiated PTFE film was immersed into hydrocarbon group reactive monomer-containing styrene solution, which contains (in mass parts) styrene (40), divinyl benzene (2), azobis isobutyro nitrile (0.01) and n-hexane (40). The PTFE film was then reacted at 60degreesC for 15 hours. The film was removed from the solution, washed with toluene and acetone, and dried. The grafted film was reacted with chloro sulfonic acid at 50degreesC among 1,2-dichloro ethane solvent for 2 hours. The resulting film was washed with 1,2-dichloro ethane and ion exchange water, and dried. The other side of the PTFE film was irradiated with electron beam at 80 kV. The film was immersed into a solution containing fluoro carbon-type reactive monomer (CF2=CFOCF2CF(CF3)O(CF2)2SO2F) (30), hexafluoro metaxylene (30) and azobis isobutyro nitrile (0.01). The immersed film was reacted at 50degreesC for 24 hours, and a solid electrolyte film was obtained. The obtained film had proton conductivity of 1.2 meq/g, and excellent oxidation resistance.