Disclosed is a polymer electrolyte having a protonic acid group that is excellent in thermal stability and dimensional stability. The polymer electrolyte includes copolymers having a sulfonic acid group which has a structure represented by the following formula: ##STR00001## wherein X is a divalent electron-withdrawing group; Y is an oxygen or a sulfur; Z and Q are each a direct bond, --O--, --S--, --CO--, --SO.sub.2--, --[C(R').sub.2].sub.g-- (g: integer of from 1 to 8), etc.; R, R' and R.sup.1 to R.sup.16 are each a hydrogen, a fluorine, an alkyl, a fluorine-substituted alkyl, an aryl or a nitrile; m, n, p, q, r, s, t and u are each an integer of from 0 to 4 (with the proviso that p+q.gtoreq.1); and A is a group represented by Formula (5a) or (5b) below: ##STR00002## wherein W is a divalent electron-withdrawing group; S is a protonic acid group; i is an integer of from 1 to 5; and j is an integer of from 1 to 7.