When exhaust gas temperature is low, the oxygen sensor response is dull. So, response is improved by raising the sensor temperature by passing current through the heater at a low exhaust gas temperature, such as in the immediate aftermath of the engine start, or during the warm up operation and in cutting the fuel during deceleration. Based on driving conditions and the oxygen sensor activation state, engine-ECU changes the amount of current (duty ratio) to the heater to quicken the activation of the oxygen sensor.