Various types of trivalent SC3+ cation conducting solid electrolytes of the (Sc1-xAlx)(1/3)Zr-2(PO4)(3) (0 less than or equal tox less than or equal to0.6), the Sc-1/3(Zr1-yHfy)(2)(PO4)(3) (0 less than or equal toy <1) and the (Sc0.8Y0.2)(1/3)Zr-2(PO4)(3) solid solutions were prepared and the Sc3+ ion conducting properties were compared. The optimum SC3+ ion conductivity in NASICON type solid electrolytes was successfully obtained for the (SC0.8Al0.2)(1/3)Zr-2(PO4)(3) and the Sc-1/3(Zr0.8Hf0.2)(2)(PO4)(3) solids, holding almost the equivalent NASICON crystal lattice size, and the optimization of the Sc3+ ion conduction in NASICON structure was realized.