To dissolve the ripple of the rise of the output waveform at high voltage and the wave fluctuation of the output waveform by providing a high- voltage power unit with a control means which controls the duty of the reference signal of the specified frequency, according to the change of input voltage.

A control means 3 is provided with a computation means which computes the product of the input voltage and the duty of the reference signal, and the computation means is constituted of the integrating circuit consisting of a resistor R3 and a capacitor C3. Then, the output from the computation means by the integration circuit and the set value constituted by a DC power unit E2 are compared with each other, and according to its difference, a control signal is outputted by the control signal output means constituted of a differential amplifier. By the output of the control signal, the duty of the reference signal is controlled changeably by the duty change means constituted of a PWM circuit. Hereby, the ripples of the rise of the output waveform at high voltages and the fluctuations in waveform can be dissolved.