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A planar-type NASICON thick-film CO2 sensor attached with integrated auxiliary electrode composed of NASICON, Li2CO3, and a little Pt paste is investigated. The electromotive force (EMF) values of the device in the CO2 concentration (300-750 ppm) are measured at 300-500°C under dry as well as humid conditions. The relationship between EMF values and logarithm of CO2 concentration is linear with the number of reaction electrons n=1.9, 2.0 at 400 and 500°C, respectively, which are excellent agreement with the theoretical values (n=2.0). Compared with the conventional sintered pellet sensor fitted with binary carbonate (Li2CO3-BaCO3) as auxiliary electrode, the sensor showed smaller cross-sensitivity against humidity and faster CO2 response time as short as 50 s for 90% saturation. And most important, the EMF of the sensor is more stable than that of the conventional sensor during the heating-cooling process and without any degradation. The long-term stability of the sensor is also perfect.