TECHNOLOGY FOCUS - POLYMERS - Preferred Polymer: The fluoropolymer is a copolymer containing tetrafluoroethylene unit and unit of formula (I) CF2=CF(OCF2CFX)m-Op-(CF2)nA.X = fluorine or tetrafluoromethyl group;A = sulfonic acid group or its precursor acid group;m = integer of 0-3;n = integer of 0-12;p = 0 or 1, provided when n = 0, p = 0. The copolymer is obtained by copolymerization of the comonomer of perfluoro unsaturated compound which has two double bonds, or perfluoroethene or perfluoro(alkylvinyl ether) which has a bromine atom, carboxylic acid group or carboxylate group, cyano group, or cyanate group. EXAMPLE - Tetrafluoroethylene (in mols) (0.4), perfluoro (alkylvinyl) ethers CF2=CFOCF2CF(CF3)O(CF2)2SO2F (0.134) and CF2=CFO(CF2)3COOCH3 (0.02), were copolymerized at 70degreesC in the presence of azobisisobutryonitrile as an initiator. A copolymer with an ion exchange capacity of 1.27 milliequivalents/g of dry resin, was obtained. The copolymer was hydrolyzed with an aqueous solution containing 15% potassium hydroxide and 30% dimethyl sulfoxide followed by immersion to 1 mol/l hydrochloric acid at room temperature for 16 hours and washing with water. The copolymer was then dissolved in mixed solvent of ethanol and 1,3-dichloro-1,1,2,2,3-pentafluoropentane propane at a mass ratio of 80:20. An ion exchange polymer solution which consists of perfluorocarbon polymer containing sulfonic acid group and crosslinked carboxylic acid group, was obtained. Coating liquid of ion exchange polymer solution and platinum carrying carbon at a mass ratio of 1:3, was prepared. The coating liquid was applied on a carbon cloth, and dried. A gas diffusion electrode with gas diffusion layer was formed. A film made of copolymer with ion exchange capacity of 1.1 milliequivalents/g of dry resin of tetrafluoroethylene and perfluoro (alkylvinyl) ether CF2=CFOCF2CF(CF3)O(CF2)2SO2F, was formed. The film was pinched between gas diffusion electrodes and pressed followed by heating at 180degreesC for 2 hours. The ion exchange polymer in the electrode was crosslinked. The film negative electrode contact piece was produced. The fuel cell containing the film negative electrode contact piece, was manufactured. The terminal voltage at a current density of 1A/cm2 and after 1000 hours, was 0.58V and 0.57V, respectively.