To obtain a perfluorosulfonic acid polymer chemically stable, excellent in oxidation resistance and high in ion-exchanging capacity, a membrane thereof, their production processes, and further a juncture of electrode/membrane for a solid polymer electrolyte fuel cell using the polymer and the membrane.

This perfluorosulfonic acid polymer is a perfluoropolymer having a sulfonic acid-derived group where the capacity of the sulfonic acid-derived group is ≤1,500 g/equivalent, containing no ether bond nor C-X bond (X is a halogen atom except fluorine) in the molecule. The membrane is made from the perfluorosulfonic acid polymer. The juncture of electrode/membrane for the solid polymer electrolyte fuel cell uses the polymer and the membrane.