A CO2 gas sensor using a combined solid electrolyte of a Na+ conducting Na2O-Al2O3-4SiO(2) glass and an yttria-stabilized zirconia was prepared, and its sensing properties in dry and wet gases were investigated using a variable flow rate of the test gases. The glassy electrolyte in the sensor was suitable as a CO2 sensing material because of its high density and good contact between the electrolytes. In a dry atmosphere, excellent results were obtained for the CO2 gas concentration dependence of the electromotive force (EMF), expressed by EMF (mV) = 547.5 - 74.0 log [C-CO2/ppm] at 470 degrees C in which the sensitivity of 74.0 mV/decade was in fair agreement with the theoretical value based on a two-electron electrochemical reaction. In the case of wet CO2 gas, the sensitivities and response time were depressed even for a high CO2 gas concentration. The decomposition of Na2CO3 into sodium oxide on the Pt electrode was accelerated in the presence of H2O. The reason for the depression of the sensing properties in a humid condition could be ascribed to its reformation reaction as a side reaction.