Tetravalent Ge4+ ion conduction in solids was first demonstrated by selecting a NASICON-type GeNb(PO4)(3) solid and partially replacing P5+ ion in GeNb(PO4)(3) with Si4+ ion to expand the crystal lattice, which allows for realization of a Ge4+ ion with high valence and electronegativity. Among the Ge1+x/4NbP3-x-SixO12 solids prepared, it was found that Ge41/40NbP2.9Si0.1O12 (x = 0.1) solid showed the highest Ge4+ ion conductivity.