PURPOSE: To attempt improvement in the temperature controllability of a heater by finding a difference between a measured temperature and a preset temperature in a differential amplifier, then employing a comparator to compare the difference with a reference signal given from a reference signal generator circuit and adjusting the level of power supplied to the heater according to the difference.

CONSTITUTION: A heater control circuit obtains as the output B of a differential amplifier 3 a difference in voltage between the output of a converter 1 which converts the temperature of an object of measurement into its corresponding voltage and the output of a reference voltage generator circuit 2. Then a comparator 6 compares the output B with the output reference signal A of a reference signal generator circuit 5, synchronized with the output of an oscillator 4. Although power supplied to a heater 8 via a switch circuit 7 in synchronization with the output C of this comparator 6 is intermittent, it becomes continuous if the oscillating frequency of the oscillator 4 is increased. The level of power supplied to the heater can thus be adjusted according to a large or small value of difference between a measured temperature and a preset temperature to control operation of the heater in a phaseless manner so as to decrease the number of times that the heater causes overrun.