A solid-state electrochemical cell of the type O-2(air), Pt/La0.9Sr0.1MnO3/Li2CO3(+ 5 mol% Li3PO4+6 mol% Al2O3)/Au, CO2, O-2 was composed to determine CO2 concentration, where Li2CO3, a lithium ion conductor, was used as an electrolyte, and the perovskite-type oxide (La0.9Sr0.1MnO3)/O-2-electrode as a reference electrode. The electromotive force (EMF) of the cell was found to be proportional to the logarithm of CO2 partial pressure in CO2/O-2/N-2 gas mixtures at temperatures between 300 and 400 degrees C. The EMF responded to changes of CO2 partial pressure within 1 min at 400 degrees C. The sensitivity to CO2 of this cell was not affected by coexistence of O-2, and the EMF remained constant after the first 15 days. The mechanism for sensing CO2 is discussed.