PURPOSE: To measure oxygen concn. with good accuracy, by bringing an oxygen detection element to a predetermined temp. by controlling the heater of an oxygen sensor used in controlling the air/fuel ratio of an engine on the basis of the detection value of an engine load state and the detection value of the change ratio thereof.

CONSTITUTION: A load state is detected from an air sucking amount and a cooling water temp. by an engine load state detection means 32 for detecting a combustion state. However, because the combustion state and the air sucking amount are not correlated at the time of the excessive operation of the engine, an engine load detection value is inputted to an engine load change ratio detection means 39 to operate a change ratio. Next, this load detection to an objective power setting means 22 and the load detection value is corrected by the change ratio. In the next step, this output is inputted to a heater control means 21 and small power is inputted to the heater of an oxygen sensor 11 when the load detection value is large which large power is inputted thereto when small to make the temp. of an oxygen detection element part rational. Therefore, the oxygen detection element part is held to a predetermined temp. and, as a result, oxygen concn. can be detected with good accuracy and an air/fuel ratio can be controlled normally.