The gas sensor passes at least three levels of current through the heater element, stepwise and continuously for a predetermined time. The calculator loads voltages across the heater element for respective current values after a predetermined elapsed time, to obtain temperature from a voltage across the heater element, for the minimum amount of current. Further, from the temperature, zero-point correcting formulas, and sensitivity correcting formulas, the calculator corrects the voltages across the heater element to obtain respective normalized outputs. From the difference between these normalized outputs, the calculator obtains humidity to correct at least one of the normalized outputs with the humidity and the humidity correcting formula. In such a way, the calculator obtains the concentration of the gas to be sensed.