An oxygen sensor is mounted in an exhaust path of an internal combustion engine. The status of an exhaust gas is detected in accordance with the output of the oxygen sensor. The oxygen sensor incorporates a heater for heating an element of the sensor. In a region in which the temperature of the sensor element is below 300° C., an adsorbable species becomes adsorbed. In a region in which the exhaust pipe temperature is above 80° C., the adsorbable species becomes adsorbed remarkably. Power supply control is continuously exercised over the heater so as to maintain the sensor element at a temperature of 300° C. or higher until the exhaust pipe temperature drops below 80° C. after an internal combustion engine stop. The power supply to the heater is shut off after the exhaust pipe temperature drops below 80° C.