A solid polymer electrolyte includes a polymer having a sulfonic group and shows high proton conductivity in a wide range of temperatures and humidities and is excellent in hot water resistance and chemical stability. A proton conductive membrane is composed of the electrolyte. The polymer electrolyte includes a polymer having a structural unit represented by the following formula (1a) or (1b): ##STR00001## wherein A is a divalent electron-withdrawing group, B is a divalent electron-donating group or a direct bond, Ar is an aromatic group, Z is an oxygen or a sulfur atom, X is a hydrogen or a fluorine atom, j is an integer of 1 or greater, k is an integer of from 1 to 4, l is an integer of from 0 to 10, m is an integer of from 1 to 10, and n is an integer of from 1 to 8.