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: 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—, —SO2—, —[C(R′)2]g— (g: integer of from 1 to 8), etc.; R, R′ and R1 to R16 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≧1); and A is a group represented by Formula (5a) or (5b) below: 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.