An excessive overshoot preventing unit (16) is connected to a loop in which a command voltage Vf according to a difference between: a voltage obtained by dividing an output voltage Vout as a high voltage; and a control voltage V cont set from the outside is obtained and fed back to each of drive circuits (3 and 5) of a positive voltage generating unit (2) and a negative voltage generating unit (4). The excessive overshoot preventing unit (16) clamps the command voltage Vf at a voltage value according to the control voltage Vcont. An overshoot that occurs in the voltage generating unit (2 or 4) at the time of polarity switching mainly depends on a circuit constant, and hence the amount of overshoot is excessive in the case of a low-voltage output even if the amount of overshoot is optimal in the case of a rated output. To deal with this, in this power unit, the overshoot of the command voltage Vf in suppressed by the excessive overshoot preventing unit (16), and hence the output voltage can be promptly settled to a target voltage even in the case of a low-voltage output.