TECHNOLOGY FOCUS - POLYMERS - Preferred Properties: The generation amount of fluoride ion produced by immersing fluorocarbon group electrolyte polymer in water (20 ml) containing hydrogen peroxide (1 mass%) and bivalent iron ion (2 ppm) at 80degreesC, is 0.001 mass% or less with respect to original polymer mass. Preferred Process: The processed electrolyte polymer is dissolved in solvent and then formed into film by cast film formation method. EXAMPLE - Fluorine compound (CF2=CFOCF2CF2SO2F) (in g) (66.9) and 1,1,1,2,3,4,4,5,5,5-decafluoro pentane (HFC43-10mee) (40) were added into stainless-steel-made autoclave (200 ml). Then, HFC43-10mee solution of fluorine compound ((CF3CF2CF2COO)2) was added as initiator, and the container was substituted with nitrogen. Then, tetrafluoroethylene was introduced and stirred at 25degreesC. The solvent and remaining monomers were removed, and remaining solid substance was washed by HFC43-10mee, and vacuum-dried at 110degreesC to obtain perfluoro electrolyte polymer (A) (13.7) having CF2=CFOCF2CF2SO2F monomer unit and tetrafluoroethylene monomer unit. The obtained perfluoro electrolyte polymer (A) was subjected to fluoridation process using fluorine gas which was diluted to 20 mol% with nitrogen to obtain perfluoro electrolyte polymer (B). The obtained polymer (B) was pressed to form a film of 50 microns thickness. The film was subjected to saponification using potassium hydroxide/dimethyl sulfoxide/water in mass ratio of 30:15:55. The film was then immersed in sulfuric acid, washed with water and dried to obtain sulfonic acid type perfluoro electrolyte film (A). The perfluoro electrolyte film (A) was immersed in 30 mass% hydrogen peroxide aqueous solution containing bivalent iron ion (50 ppm) for 10 hours. The film was further immersed in sulfuric acid, washed with water and dried, to obtain perfluoro electrolyte film (B). The obtained perfluoro electrolyte film (B) (180 mg) was dissolved in ethanol/water in mass ratio of 50:50 at 140degreesC, to obtain perfluoro electrolyte polymer solution (A). The obtained solution (A) was cast on a Petri dish and dried to form perfluoro electrolyte film of 2 microns thickness. The perfluoro electrolyte film eluted 0.00010 mass% of fluoride ion, when measured by Fenton test. The driving ability of fuel cell was improved by using the obtained perfluoro electrolyte film.