A solid polymer electrolyte is made up of a polymer compound having a hydrocarbon aromatic group in the backbone thereof and including a side chain expressed by FORMULA 1: CH.sub.2.sub.nSO.sub.3H FORMULA 1 wherein "n" is 1, 2, 3, 4, 5, or 6. The solid polymer electrolyte may be incorporated into a membrane and may be used in a solution for covering an electrode catalyst.