A solid-state electrochemical cell of the type Pt/LiCoO2-5 mole percent (m/o) Co3O4/Li2CO3 (+5 m/o Li3PO4 + 6 m/o LiAlO2)/Au, Co-2 O-2, was composed for determining CO2 concentration, where Li2CO3, a lithium ion conductor, is an electrolyte, and LiCoO2-Co3O4 is used as the solid reference electrode. Electromotive force (EMF) of the cell depended logarithmically on the CO2 partial pressure in CO2/O-2 gas mixtures at temperatures between 350 and 400 degrees C. EMG reached a constant value with 1 min after the change of CO2 partial pressure at 400 degrees C. The sensitivity to CO2 of this cell was not affected by coexistence of water vapor. The sensor worked stably during a test period of 30 days. The sensing mechanism of CO2 was discussed together with an explanation to the stability of this sensor.