PURPOSE: To stabilize the element temp. of an O2 sensor by providing an on-off region of a heater according to the parameter quantity of an operating condition.

CONSTITUTION: A means for detecting the parameter quantity of the operating condition detects the parameter quantity of the prescribed operating condition of an engine, for example, intake air quantity Q of the engine. The 1st discriminating means discriminates whether the detected quantity Q is above the 1st prescribed value A or not and the 2nd discriminating means discriminates whether the quantity Q is below the 2nd prescribed value B smaller than the value A or not. As a result, a heater off means turns off the conduction of electricity to a heater when the detected quantity Q is above the value A. A heater on-off means turns on and off the conduction of electricity to the heater at prescribed time intervals when the detected quantity is below the value A, i.e., exceeds the value B. A heater on means turns on the conduction of electricity to the heater when the detected quantity Q is below the value B.