To stably control an inside temperature to a set temperature without being affected by the change an external air temperature by comparing the output of a differential amplifying means with that of sawtooth wave generating means and controlling the ON/OFF of electrification to a heater based on this compared output.

A controller 1 generates the output in proportion to the difference between the inside temperature detected by an inside sensor 6 and the temperature set by a setting circuit 11 through a differential amplifier circuit 8 and changes the amplification factor outputted by that differential amplifier circuit 8 through an external air temperature correction circuit 23 based on the output of an external air temperature sensor 13. Then, based on the output of that external air temperature sensor 13, the output of the differential amplifier circuit 8 is compared with the output of a sawttoth wave generating circuit 14 for generating sawtooth waves by a comparator circuit 12 and based on the output of this comparator circuit 12, the ON/OFF of electrification to a heater 17 is controlled by an SSR 16. Thus, the inside temperature is controlled to the set temperature without being affected by the change of the external air temperature.