A gas sensor includes a first sensor part that can detect the concentration of the mixture of a first gas and a second gas, a second sensor part having higher detection sensitivity with respect to the second gas than with respect to the first gas, and a signal processing circuit that subtracts the concentration of the second gas detected by the second sensor part from the mixture concentration detected by the first sensor part to derive the concentration of the first gas. The concentration of the second gas detected by the second sensor part is subtracted from the concentration of the mixture concentration detected by the first sensor part, so that it is possible to cancel the influence of the second gas which is miscellaneous gas to thereby work out a correct value of concentration of the first gas to be detected.