Provided is solid electrolyte utilizing a composite oxide of a RP-type structure, that is useful for achieving strong electromotive force and enhanced current-voltage characteristics of a fuel battery, has enhanced ion conductivity and sufficiently inhibited electronic conductivity, and is capable of intercalation of a large amount of water or hydrogen groups, as well as a solid electrolyte membrane, a fuel battery cell, and a fuel battery. The solid electrolyte and the solid electrolyte membrane of the present invention has been obtained by subjecting a particular composite oxide of a RP-type structure or a membrane thereof to a treatment of at least one of hydroxylation and hydration, and has a property that the mass determined by TG measurement at 400° C. is less than that at 250° C. by not less than 4.0%.