Abstract: A lithium ion-conductive solid electrolyte consisting of a compound represented by the chemical formula, Li.sub.1+x M.sub.x Ti.sub.2-x (PO.sub.4).sub.3 (M is at least one element selected from the group consisting of Fe, Al and rare earth elements and x is a number of 0.1 to 1.9), a compound represented by the chemical formula, Li.sub.1+y Ti.sub.2 Si.sub.y P.sub.3-y O.sub.12 (Y is a number of 0.1 to 2.9), or a compound obtained by mixing a compound represented by the chemical formula, LiTi.sub.2 (PO.sub.4).sub.3 with a lithium compound other than LiTi.sub.2 (PO.sub.4).sub.3 and sintering the resulting mixture. Said lithium ion-conductive solid electrolyte can be produced by subjecting to solid phase reaction a mixture of a titanium oxide, a lithium salt, a phosphoric salt and M.sub.2 O.sub.3 (M is at least one element selected from the group consisting of Fe, Al and rare earth elements) or a silicon oxide or a mixture of LiTi.sub.2 (PO.sub.4).sub.3 and other lithium compound.