To detect a heater error before the upper limit temperature at which overheat safety device (thermostat) works and to avoid damages to peripheral parts is reached.

This controller is provided with a thermistor 5 detecting the temperature of a heater 2 and a timer 9a counting a prescribed time starting from the driving of the heater. Then, heater temperature at the time of starting the driving of the heater is detected, and the prescribed time counted by the timer is changed to a shorter time as the heater temperature at driving becomes higher. If the heater temperature detected when the prescribed time counted by the timer elapses does not reach target temperature, it is decided that there is a heater error, and the heater is stopped.