PURPOSE: To control a heater by comparing the slice levels provided within a range of the width at which the output of an exhaust gas sensor changes at a control target temp. and the difference between the max. value and min value of the output of the exhaust gas sensor and detecting the temp. from the output of the exhaust gas sensor.

CONSTITUTION: The exhaust gas sensor is constituted of a metal oxide semiconductor 4 and the heater 6. Resistors 10, 12 which divide the voltage of a power supply 2 to provide the slice levels S, L and a switching transistor 14 for controlling the heater 6 are provided; further, a comparing means 16 for detecting the air-fuel ratio of the exhaust gas by comparing the sensor output and the slice levels is provided. The slice levels S, L are set at the center of the output on the rich side and lean side at the control target temp. and the heater 6 is so controlled that the difference between the output of the rich side and the slice levels is equalized to the difference between the output on the lean side and the slice levels.