The gas sensing properties of Zr-added and pure CaFe2O4 powders for CO2 in air were examined in the temperature range of 250–450 ℃. The semiconductor-type gas sensor made from pure CaFe2O4 powder showed a fairly good response to CO2. Furthermore, the addition of a small amount of Zr into CaFe2O4 powder was found to be effective for enhancing the CO2 response of the present gas sensor. It was also found that the gas response, defined by the ratio of the resistance in air and that of the target gas reached maximum at the operating temperature of 300 ℃. The gas response of the Zr-added CaFe2O4-based sensor at 300 ℃ was estimated to be 2.5 times higher than that of the sensor made from pure CaFe2O4 powder. However, the 90% response time of the Zr-added CaFe2O4-based sensor was much quicker at 350 ℃ than that at 300℃. Thus, the optimal gas sensing performance of the Zr-added CaFe2O4-based sensor is expected to be obtained at the operating temperature of 350 ℃, considering the still higher response to CO2 gas at this temperature. It is noted that the present CaFe2O4-based sensor responded reversibly as well as continuously to CO2 gas. Infrared analysis revealed that the sensing mechanism of the present CaFe2O4-based sensor is the change in the electric resistance of CaFe2O4 caused by reactive CO2 adsorption with negatively charged oxide ions (O−) resulting in the increase in the hole concentration in the base material of CaFe2O4.