TECHNOLOGY FOCUS - POLYMERS - Preferred Properties: The sulfonation equivalent of an internal layer is 120% or more with respect to the sulfonation equivalent of the adjoining film surface layer. The surface of which sulfonation equivalent increases from both surface to 5 microns wide region discontinuously exists. The surface layer and internal layer are separated by the surface. Alternately, the sulfonation equivalent of the region at a depth of 5 microns along thickness direction is 120% or more from both surface of the film with respect to sulfonation equivalent of the surface region of the film which exists in the position of 5 microns from the region. The sulfonation equivalent of the whole electrolyte film is more than 0.3 mg equivalent/g. The sulfonation equivalent of the polyelectrolyte of the region up to 1 microns is less than 3 mg equivalent/g towards both of film surfaces to interior. EXAMPLE - Bis(4-(4-(4-chlorophenyl sulfonyl)phenoxy)phenyl)sulfone (in g) (25.17), 2,5-dichloro- 4'-(4-thiophenoxy)thiophenoxy benzophenone (16.3), bis (triphenyl phosphine) nickel dichloride (1.43), sodium iodide (1.37), triphenyl phosphine (7.73) and zinc dust (11.3) were stirred in nitrogen atmosphere. Then, N-methyl-2-pyrrolidone (0.2 l) was added and polymerized at 80degreesC for 4 hours. The resulting solution was diluted with tetrahydrofuran, and polymer was recovered. The polymer was washed with methanol, vacuum dried and 95% yield of polyelectrolyte (34.7) having number average molecular weight of 40000 and weight average molecular weight of 145000 was obtained. The polymer was sulfonated using sulfuric acid. The sulfonated polymer and the polymer were dissolved in N-methyl-2-pyrrolidone at 80degreesC, and polyelectrolyte was obtained. The polyelectrolyte was applied on a glass substrate and dried at 80degreesC for 0.5 hour and polymer electrolyte film having thickness of 5 microns was obtained. The film had excellent oxidation resistance, mechanical property, durability, high temperature proton conductivity and high stability.