When a detection signal obtained from the cell of a gas sensor (S15) has reached a start determination value (specifically, when the output voltage of the cell is higher than 600 mV (S16: YES) or lower than 300 mV (S17: YES)), a pulse voltage is applied to the cell (S18), and a start-time internal resistance is obtained on the basis of the detection signal having changed as a result of application of the pulse voltage (S20). The start-time internal resistance is compared with a deterioration determination value set in advance (S21). A target resistance of the cell used in temperature control (energization control) for the heater is corrected in accordance with the result of the comparison (S28). Thus, the temperature of the cell can be stably maintained constant irrespective of deterioration of the cell.