When power begins to be supplied to a heater, open loop control is selected as the power supply control method to supply a predetermined power to the heater. After that, the control method is switched to feedback control to set the power to be supplied to the heater based on the difference between the target temperature and the actual temperature of the sensor chip if a predetermined period of time in which the difference between the target temperature and the actual temperature is expected to fall within a predetermined reference difference has passed since power began to be supplied to the heater.