Abstract: A phosphorus atom-containing fluorinated cation exchange membrane substantially constituted with the following repeating units (A) and (B), and a proton conduction type fuel cell using the membrane as a solid polymeric electrolyte are provided. One embodiment for the membrane is useful as a perfluoro cationic exchange membrane for use in electrolysis of sodium chloride even under severe electrolytic conditions of higher concentration, higher temperature and higher electric density. (in which m is 0 or 1, n is 2 or 3, X, Y represents H or C6H4SO3H and the ratio for the number of repeating units is (A)/(B)=1.5 to 15).