TECHNOLOGY FOCUS - POLYMERS - Preferred Polymer: The polymer is a graft copolymer comprising segment (B) as principal chain and segment (A) as side chain. The ion exchange group contained in segment (A) is -(SO2X(SO2Rf)a)-M+. The repeating unit based on perfluoro monomer, has formula (1 or 2).M+=ammonium ion optionally containing H+, monovalent metal cation or hydrocarbon substituent;Rf=perfluoro alkyl group having ether property oxygen atom;a=0-2;X=oxygen atom, nitrogen atom or carbon atom;R1=bivalent perfluoro organic group having ether property oxygen atom;andR2-R6=univalent perfluoro organic group containing fluorine atom or ether property oxygen atom, preferably R5, R6 is fluorine atomwhen X is oxygen atom, a=0. when X is nitrogen atom, a=1. When X is carbon, a=2. Preferred Fluoropolymer: The fluoropolymer which comprises segment (B) is copolymer of polytetrafluoro ethylene and ethylene-tetrafluoroethylene, tetrafluoro ethylene-perfluoro (alkyl vinyl ether)copolymer, tetrafluoro ethylene-hexafluoro propylene copolymer, polychloro trifluoro ethylene resin, ethylene-chloro trifluoro ethylene copolymer, polyvinylidene fluoride and/or polyvinyl fluoride. Preferred Material: The electrolyte material has ion exchange capacity of 0.5-3 milliequivalent/g dry resin. EXAMPLE - 20 kGy gamma-ray was irradiated on a polymer film Toyoflon 50F of thickness 50 microns under nitrogen atmosphere. The film was immersed in perfluoro compound synthesized according to the method WO03/037885 and reacted at 60degreesC for 117 hours. Washing was performed followed by drying. The obtained graft copolymer in the shape of film, was immersed in 18 mass% of potassium hydroxide aqueous solution. Heating was performed at 80degreesC for 16 hours, followed by water-washing. The film was immersed in 2 mol/L hydrochloric acid, washed and dried. The obtained electrolyte film had a specific resistance of 18 MOmega.cm and total organic carbon content of 10 ppb. The electrolyte film had excellent durability and provided fuel cell having favorable characteristics.