An O2 sensor has a sensor element, which includes a solid electrolyte layer and a pair of electrodes. The solid electrolyte layer is held between the electrodes, which includes an atmosphere side electrode and an exhaust side electrode. A constant current circuit is installed in an electric path, which connects between the atmosphere side electrode and a ground, to induce a flow of a predetermined constant electric current between the electrodes through the solid electrolyte layer. When the sensor element generates an electromotive force, the constant current circuit conducts an electric current, which is generated while using the electromotive force of the sensor element as an electric power source, to induce the flow of the predetermined constant electric current between the electrodes in an electromotive force range, which is equal to or larger than an electromotive force of the sensor element generated at a stoichiometric air-to-fuel ratio.