TECHNOLOGY FOCUS - POLYMERS - Preferred Polymer: The fluorine-containing polymer having sulfonic acid group, is copolymer which contains repeating unit derived from monomer (A) which is perfluoro monomer, and repeating unit derived from monomer (B) of formula: CF2=CF-(OCF2CFY)m-Op-(CF2)n-SO3H.Y = fluorine atom or trifluoromethyl;n = 0-3;p = 0 or 1; andn = 1-12.Preferred Monomer: The monomer (A) is chosen from perfluoro (3-butenyl) vinyl ether), perfluoro (2,2-dimethyl-1,3-dioxole), perfluoro (1,3-dioxole), 2,2,4-trifluoro-5-trifluoro methoxy-1,3-diaxole and perfluoro (2-methylene-4-methyl-1,3-dioxolane), preferably perfluoro (2,2-dimethyl-1,3-dioxole), perfluoro (3-butenyl) vinyl ether) and perfluoro (2-methylene-4-methyl-1,3-dioxolane). The monomer (B) is perfluoro (3,6-dioxa-4-methyl-7-octene) sulfonic acid or perfluoro (3-oxa-4-pentene) sulfonic acid.Preferred Property: The perfluoro polymer having sulfonic acid group, has ion-exchange capacity of 0.5-2 mEq/g.Preferred Method: The fluorine-containing polymer having aliphatic ring in principal chain and sulfonic acid group, is molded to obtain polymer electrolyte film. EXAMPLE - Perfluoro (2,2-dimethyl-1,3-dioxole) (PDD) (in g) (26), fluorine-containing compound of formula, CF2=CFOCF2CF (CF3) OCF2CF3SO2F (PSVE) (127.8), and methylene-containing compound of formula, (CH3)2 CHOC (=O) OOC (=O) OCH (CH3)2 (IPP) (0.46), were heat-stirred at 40degreesC under 0.3 MPa in nitrogen atmosphere and cooled. The obtained precipitate was washed with hexane and vacuum-dried at 80degreesC for 16 hours after filtration, to obtain white polymer (41.6) having ion-exchange capacity 1.31 mEq/g and number average molecular weight of 33000 based on polymethyl methacrylate conversion. The obtained polymer (10) and fluorine gas (20 volume%) diluted with nitrogen gas, were reacted. The resulting mixture was maintained at 160degreesC for 30 minutes and further reacted with alkali, to obtain acid group-containing polymer having softening point of 150degreesC. The resulting polymer was dried and dissolved in ethanol, to obtain 10% transparent solution. A cast film with thickness of 200 microns was obtained from the solution and heated at 160degreesC for 30 minutes. A fuel cell formed using the resulting film, had excellent durability and large industrial value.