Provided is a CO sensor which comprises a solid electrolyte 11 having oxygen ion conductivity and a standard electrode 15 and a sensing electrode 16 for measurement of carbon monoxide and at least one standard electrode 15 and sensing electrode 17 for measurement of at least one different gas other than carbon monoxide, especially sulfur dioxide, these electrodes being formed on at least a part of the surface of the solid electrolyte 11. This CO sensor is excellent in selectivity for carbon monoxide, can exclude the influence of the coexisting sulfur dioxide and oxygen on the carbon monoxide measured value, and, besides, can be used at high temperatures. A method for making the sensor and a method of using it are also provided.