TECHNOLOGY FOCUS - INORGANIC CHEMISTRY - Preferred Component: The cerium atoms are contained as cerium ions or cerium oxide. The cerium ion is 0.3-20 mol% of -SO3- groups contained in the ion exchange membrane. The cerium oxide is present at 0.3-80 mass%.    TECHNOLOGY FOCUS - ORGANIC CHEMISTRY - Preferred Component: The acidic groups are sulfonic acid groups. EXAMPLE - An autoclave was charged with (CF3)3COOC(CF3)3 (5.6 mg), and CF2=CFCF2CF2SO2F (63.75 g), and followed by deaeration under cooling with liquid nitrogen. The reaction was increased to 100 degrees C, tetrafluoroethylene was introduced to the system, and was maintained at 0.59 MPaG. A nitrogen gas was added to adjust at 1.05 MPaG. Then, the reaction was increased to 130 degrees C and maintained at 1.3 MPaG. After stirring at 130 degrees C for 17 hours, the gas in the system was purged, and the autoclave was cooled to terminate the reaction. The product was diluted with CClF2CF2CHClF, and CH3CCl2F was added to coagulate the polymer, which was subjected to filtration. Then, the polymer was stirred in CClF2CF2CHClF, re-coagulated by CH3CCl2F, and dried under reduced pressure at 80 degrees C overnight. The formation amount was 2.5 g. To the obtained polymer, a fluorine gas diluted to 20% with a nitrogen gas was introduced to 0.3 MPaG, and the polymer was maintained at 180 degrees C for 4 hours. Then, a membrane having a thickness of 50 mu m was obtained by hot pressing. For hydrolysis, first, the membrane was immersed in a solution of potassium hydroxide (KOH) in water and dimethyl sulfoxide as solvents (KOH/water/dimethyl sulfoxide=15/55/30 mass ratio) and then immersed in hydrochloric acid for conversion to an acid form, and the membrane was washed with ultrapure water. The ion exchange capacity of the obtained ion exchange membrane was measured by titration and as a result, it was 1.13 meq/g dry resin. An electrolyte membrane was sandwiched between the cathode catalyst layer and the anode catalyst layer, followed by hot pressing at 120 degrees C and 3 MPa for 2 minutes to bond the catalyst layers to the membrane, and each substrate film was separated to obtain a membrane/catalyst layer assembly having an electrode area of 25 cm2.