TECHNOLOGY FOCUS - POLYMERS - Preferred Compound: The epoxy group-containing compound (D) is a homopolymer or copolymer (E) of unsaturated monomer having an epoxy group, preferably an epoxy resin (F). The high molecular compound (A) having ion exchange group is a perfluorocarbon high molecular compound having ion exchange group. The perfluorocarbon high molecular compound having ion exchange group is of formula: -(CF2CX1X2) a-(CF2-CF(-O-(CF2-CF(CF2X3))b-Oc-(CFR1)d-(CFR2)e-(CF2)f-X4))gX1-X3=halogen or 1-3C perfluoro-alkyl;a=greater than or equal to0 and less than 1;g=more than 0 and less than or equal to1;a+g=1;b=0-8;c=0 or 1;d-f=0-6;d+e+f=not equal to 0;R1,R2=halogen, 1-10C perfluoro alkyl or fluorochloro alkyl;X4=COOZ, SO3Z, PO3Z2 and/or PO3HZ;Z=hydrogen atom, alkali-metal atom, alkaline-earth metal atom and/or amines such as NH4, NH3R, NH2R2, NHR3, NR4;andR=alkyl or arene group.Preferred Method: The polyelectrolyte composition contains an epoxy-denatured polysulfone (G) formed by reacting polysulfone resin (C) and epoxy resin (F). EXAMPLE - Precursor polymer (in weight parts) (90) obtained from tetrafluoroethylene and compound of formula: CF2=CFO(CF2)2-SO2F, and having melt index of 3 and equivalent weight of 730, polyphenylene sulfide (7) containing methylene chloride of 0.7 wt.% and having melt viscosity of 50 Pa.seconds at 300degreesC and load of 196 N/cm2, N-660 (RTM: cresol novolak type epoxy resin) (1) and polysulfone (3) having number average molecular weight of 26000 were added and melt-kneaded. Then, the kneaded product was molded to a film having thickness of 50 microns. The film was contacted with an aqueous solution containing potassium hydroxide (15 wt.%) and dimethyl sulfoxide (30 wt.%) at 60degreesC for 4 hours and alkali hydrolysis was performed. Then, the film was immersed into water at 60degreesC for 4 hours and then in aqueous 2N hydrochloric acid solution at 60degreesC for 3 hours. Then, the film was water-washed using ion exchange water and dried to obtain proton exchange film having equivalent weight of 795. The proton exchange film had proton conductivity of 0.24 S/cm, cross leak amount of 5.9x10-13 ml.cm/cm2/sec/Pa, fluorine elution rate of 0.013 micrograms/hour/cm2 and 0.014micrograms/hour/cm2 at 200 hours and 500 hours, respectively and tension elastic modulus of 4136 N/cm2. The fuel cell using the proton exchange film had excellent durability.