PURPOSE: To use the same heater even against a different voltage, by recognizing a difference of an input voltage by a microprocessor, and converting a parameter on its program.

CONSTITUTION: A temperature in a heater part 3 is detected by a detecting part 4, brought to a digital conversion by an analog-to-digital converting part 5, and a result of conversion is inputted to an input port of a microprocessor 1. On the heater hand, by a signal from a voltage switching part 6, whether the present voltage level is, for instance, 120V or 240V is recognized. Subsequently, by driving a relay part 2, a supply of AC power to the heater part 3 is started. The microprocessor 1 intermits the AC relay part 2 under a different control condition at every power supply voltage, and controls a heating temperature of the heater part 3 to a common set temperature, inrespective of whether the power supply voltage value is high or low.