A compact carbon dioxide gas sensor was fabricated by applying a trivalent Sc3+ cation conducting Sc-2(WO4)(3) solid electrolyte and an oxide anion conducting stabilized zirconia with Li2CO3 as an auxiliary electrode. The combination of an oxide ion conductor which covers one of the trivalent ion conducting Sc-2(WO4)(3) solid surfaces is greatly effective for obtaining a stable output response. The time necessary for a 90% response was within several minutes, and the response exactly coincided with the theoretical Nernstian equation, and a 1:1 linear relation was observed between the sensor output and the logarithm of the carbon dioxide gas concentration. Quantitative CO2 gas monitoring was successfully realized with a rapid, continuous, reproducible and selective response with a long-term stability.