A sensor for detecting oxygen concentration in exhaust gas or an air-fuel ratio provided with a solid electrolyte body, an exhaust gas side electrode being disposed on one side of the solid electrolyte body and being in contact with the exhaust gas, an atmosphere side electrode being disposed on the other side of the solid electrolyte body and being in contact with the atmosphere, and an electric circuit applying a reference voltage between these electrodes is arranged in an engine exhaust passage. The sensor for detecting the oxygen concentration in the exhaust gas or the air-fuel ratio has a characteristic in which an output current continues to increase without having a limiting current region when the voltage applied between the electrodes is increased while the air-fuel ratio is constant. The air-fuel ratio is controlled based on the output current of the sensor for detecting the oxygen concentration in the exhaust gas or the air-fuel ratio.