NASICON) disks with sensing electrode materials, such as Au, indium tin oxide (ITO), and auxiliary phase (nitrate or carbonate). Their NO2 and CO2 sensing properties were examined under 30% relative humidity (RH) in the temperature range of 30 to 100℃. When the NO2 sensor was operated in the range of 2 to 7 ppm NO2 at 30℃, the LISICON sensor attached with ITO-NaNO2 had the highest sensitivity to NO2 and the electromotive force (EMF) change was 41.4 mV. At 30℃ under 30% RH, the EMF values of the LISICON sensor attached with ITO-NaNO2 were proportional to the logarithm of NO2 concentration. Also, when the CO2 sensor was operated in the range of 250 to 2,500 ppm CO2 at 30℃, the LISICON sensor attached with ITO-Li2CO3-BaCO3 had the highest sensitivity to CO2 and the EMF change was 20.0 mV. At 30℃ under 30% RH, the EMF values of the LISICON sensor attached with ITO-Li2CO3-BaCO3 were correlated linearly with the logarithm of CO2 concentration. Subsequently, to examine gas selectivity, the NO2 selectivity of the sensor attached with ITO-NaNO2 and the CO2 selectivity of the sensor attached with ITO-Li2CO3-BaCO3 were evaluated at 30℃ under 30% RH. As a result, it was found that the LISICON and NASICON sensors attached with ITO-NaNO2 were fluctuated by the change in CO2 concentration. On the other hand, although the NASICON sensor attached with ITO-Li22CO3-BaCO3 was affected by the NO2 concentration change, the LISICON sensor attached with ITO-Li2CO3-BaCO3 showed a high CO2 selectivity. From thermodynamic calculations, it was considered that the chemical instability of NASICON and NaNO2 under CO2 led to the degradation of NO2 gas selectivity in the ITO-NaNO2-attached sensor, whereas that of NASICON under NO2 caused the degradation of CO2 gas selectivity in the ITO-Li2CO3-BaCO3-attached sensor.