A combined gas sensor of the invention is equipped with a second oxygen sensing electrode 5 active only to oxygen, which is disposed on a solid electrolyte substrate 3a such that it is directly exposed to detection gases. Sensor disorder is detected by measuring an electromotive force (signal output 2) generated between a reference electrode 7 and the second oxygen sensing electrode 5 as an oxygen concentration in an exhaust gas atmosphere, and comparing and judging the detected result with a signal output 1 corresponding to an electromotive force between a first oxygen sensing electrode 4 and the sit reference electrode 7, or with a signal output of an oxygen pump current.