NASICON (Na3Zr2Si2PO12)-based solid electrolyte-type gas sensors using various metal oxides-added Pt as a sensing electrode (SE) and a counter Pt electrode (RE), which were denoted as Pt(MO)/Pt-T (MO: Bi2O3, Cr2O3, CeO2 (the amount of MO loaded: 15 wt%), T: calcination temperature, 700–900°C), were fabricated, and their CO responses were investigated at 30°C and −10°C in dry air. The Pt(Bi2O3)/Pt-T and Pt(Cr2O3)/Pt-T sensors positively responded to CO, while the Pt(CeO2)/Pt-T sensor negatively responded to CO. In addition, the aging treatment (in dry air for 1.0 h and in 3,000 ppm CO balanced with dry air for 0.5 h at 400°C) increased the magnitude of CO responses of the Pt(Bi2O3)/Pt-T and Pt(Cr2O3)/Pt-T sensors. On the other hand, the aging treatment decreased the magnitude of CO response of the Pt(CeO2)/Pt-T sensor.