A CO sensor includes a solid electrolyte substrate, a sensing electrode, and a reference electrode, and outputs electromotive forces in accordance with CO concentrations. The sensing electrode and the reference electrode are provided on the same surface of the solid electrolyte substrate. The sensing electrode contains a metal oxide such as Bi2O3 that generates a positive electromotive force response when coming into contact with CO. The reference electrode contains a metal oxide such as CeO2 that generates a negative electromotive force response when coming into contact with CO.