Among the trivalent ion conductor series of Sc₂(WO₄)₃ type and NASICON type that we have reported, the highest ion conductivity was obtained for the trivalent Al³⁺ ion conducting NASICON type (Al_xZr_1-x)_4/(4-x)Nb(PO₄)₃ solid electrolyte and the value enters the area between yttria stabilized zirconia (YSZ) and calcia stabilized zirconia (CSZ) of well known commercialized high oxide anion conductors. The enhancement of both the ion conductivity and the mechanical strength was simultaneously realized by adding B₂O₃ as a sintering agent during the sintering procedure. The Al³⁺ ion conducting (Al_xZr_1-x)_4/(4-x)Nb(PO₄)₃ solid electrolyte with the B₂O₃ additive was combined with YSZ, and the refractory oxide based solid was set on the (Al_xZr_1-x)_4/(4-x)Nb(PO₄)₃ solid surface as the individual auxiliary electrode for carbon dioxide (CO₂), nitrogen oxides (NO_X), sulfur dioxide (SO₂) gas sensing, respectively. The sensor response was rapid, reproducible and continuous with obeying the Nernst theoretical relationship.