This invention provides a heater control system applied to an air-fuel ratio sensor to prevent the sensor being cooled during an extended idling operation after high-load running. An oxygen concentration detecting element 111 of an air-fuel ratio sensor 11 is heated by a heater 112 installed in the sensor, and its temperature is controlled to about 650°C. When a radiator cooling fan functions during idling or low-speed running after high-load running, a controller 13 detects a function of the fan and increases an electric power supplied to the heater. The heater controller prevents the sensor thus being cooled by the fan and the detection accuracy of the sensor becoming worse.