Dimensional change occurring in a fuel cell electrolyte including a basic polymer after the electrolyte is doped with acid is regulated and the occurrence of wrinkles and the like is reduced. A fuel cell electrolyte comprises a basic polymer and an organic phosphonic acid expressed by the following general formula (1): ##STR00001## where R represents an alkyl, aryl, acyl, amino, phosphonic group or a derivative of the functional group, and n is 1 or 2.