Ion conduction of trivalent aluminum in the aluminum tungstate, Al-2(WO4)(3), has been quantitatively demonstrated. The ionic conduction characteristics of Al-2(WO4)(3) were investigated by means of electrolysis and electromotive force (emf) measurement of an aluminum concentration cell by using three types of aluminum-platinum alloys, Al55Pt45, Al42Pt58, and Al35Pt65, as the electrodes. The DC electrolysis data strongly support the conclusion that ionic Al3+ is the mobile species in Al-2(WO4)(3). From emf measurements, the mobile species in Al-2(WO4)(3) was quantitatively identified as aluminum ion in a trivalent state. The conducting properties of the Al-2(WO4)(3)-In-2(WO4)(3) solid solution were investigated. The Al3+ ion conduction increased with the increase of the indium content until 80 mol.% content and the solid solution of 80 mol.% shows five times as high conductivity as pure Al-2(WO4)(3). In addition, a macroscopic ionic conduction of trivalent Al3+ in solids has been directly demonstrated.