An O2 sensor includes a sensor element using a solid electrolyte layer and a pair of electrodes placed at a position to interpose the solid electrolyte layer, detects an exhaust gas from an internal combustion engine as an object of a detection, and outputs an electromotive force signal depending on an air-fuel ratio of the exhaust gas. The sensor element is connected with a constant current circuit supplying a constant current that is prescribed. A microcomputer conducts an abnormality diagnosis of an output response of the sensor element on the basis of a state of variation in an electromotive force output. Further, when the microcomputer conducts the abnormality diagnosis, the microcomputer restricts a supply of the constant current by the constant current circuit before conducting the abnormality diagnosis.