A CO2 sensor in which the main component of an electrolyte was Li2CO3 as a lithium ion conductor was constructed as O-2, CO2, Au(1)/electrolyte/Au(2), CO2, O-2, where an electrolyte consists of Li2CO3, Li3PO4 and Al2O3, and (1) and (2) denote a probe and a reference gas, respectively. A simulated air (79% N-2 + 21% O-2) containing 1000 ppm CO2 was used as the reference gas, O-2(2) + CO2(2), and simulated airs containing CO2 in the range between 10 and 1000 ppm as sample gases, O-2(1) + CO2(1). The electromotive force (EMF) of the cell was examined at temperatures between 300 and 600 degrees C. The EMF was dependent on oxygen partial pressure as well as CO2 partial pressure, and was expressed by the Nernst equation for both the gases. When the oxygen partial pressure was kept constant, the cell functioned as a CO2 sensor. The response of the sensor was quite rapid when water vapor was not contained in the sample gases. The EMF did not depend on the existence of water vapor.