In a CO gas sensor composed of a CO gas sensing electrode 3 being made of Au or an Au alloy and a reference electrode 2 electrically connected to at least part of a surface of a solid electrolyte having an oxygen ion transfer property, which is superior in CO gas selectivity, in particular, reducing the affect of the co-existing oxygen to CO, and which works with a high precision at a high temperature, and in a CO gas measuring device using the gas sensor, the gas sensor being capable of determining a CO gas concentration by measuring a electromotive force change due to adsorption/oxidization of the carbon monoxide gas in the sensing electrode 3 when a constant current is caused to flow between the reference electrode 2 and the sensing electrode 3 or a current value caused by the oxidizing reaction of the carbon monoxide gas in the sensing electrode when a voltage is kept constant between the reference electrode 2 and the sensing electrode 3.