A membrane-electrode assembly for polymer electrolyte fuel cells which is excellent in water repellency and gas diffusivity and which exhibits a high output power density, can be obtained by using, as an electrolyte material for polymer electrolyte fuel cells, a fluoropolymer obtained by contacting a fluoropolymer which is excellent in gas diffusivity and which has alicyclic structures in its main chain and further has sulfonic acid groups, with fluorine gas for fluorination to increase water repellency and stabilize the molecule ends. The above fluoropolymer is preferably made of a copolymer comprising repeating units based on the following monomer A and repeating units based on the following monomer B (wherein Y is a fluorine atom or a trifluoromethyl group, m is an integer of from 0 to 3, p is 0 or 1, and n is an integer of from 1 to 12): Monomer A: A perfluoromonomer which gives a polymer having repeating units containing cyclic structures in its main chain by radical polymerization Monomer B: CF.sub.2.dbd.CF--(OCF.sub.2CFY).sub.m--O.sub.p--(CF.sub.2).sub.n--SO.sub.- 3H.