New tetravalent cation conductors of zirconium or hafnium niobium phosphate, MNb(PO4)(3) (M = Zr, Hf) with NASICON (Na+ Ion Super Ionic CONductors) type structure, which show the high ion conductivity among the tetravalent ion conducting solids, were synthesized. The MNb(PO4)(3) was designed by strictly selecting the structure and the comprising elements and the conductivity reaches the region of the representative divalent oxide anion conductor series. Since the MNb(PO4)(3) solids possess a considerable high ion conducting behavior, promising applications such as chemical sensors for environmental gas detection and rechargeable batteries are highly expected.