To lessen limitations caused by specifications of an A/D converter and power voltage of a control system when a high-voltage power supply unit is used by a plurality of models in common.

A high-voltage generation circuit 1 outputs high-voltage corresponding to a PWM signal to be entered. A return voltage generation circuit 2 formed by a switching means 5, and resistances R3 and R4 as formation factors to determine a ratio between output voltage and voltage of the return signal generates a return signal proportional to high-voltage, a voltage range of the return signal is controlled by switching the resistance when return voltage exceeds a specific value, and a target value of digital data corresponding to the return voltage is switched in order to change to a new target value after switching the target value of the digital data. A constant voltage control circuit 4 determines a duty of the PWN signal so that the digital data value meets the target value. When a voltage monitoring means 10 detects that the return signal exceeds the maximum digital convertible input voltage of the A/D converter 3, the resistance is automatically switched, and a voltage level of the return signal is controlled.