Power supply control system for heater used in gas sensor

A heater control system is provided for controlling the temperature of a heater used to heat a solid electrolyte-made sensor element of a gas concentration sensor up to a given temperature at which the sensor element is activated to provide a correct gas concentration output for controlling a preselected variable used in, for example, an automotive air-fuel ratio control system. The heater control system measures a resistance value of the sensor element and controls an electric power supplied to the heater as a function of the resistance value. The heater control system is designed to determine a controlled variable properly used to control the heater, thereby resulting in improved controllability of a power supply to the heater and also works to minimize an error in determining the resistance of the heater.