A signal generator generates a combined signal that is generated only in the period where supply of a pulse signal is effected, a pulse signal whose frequency is of a higher impulse repetition frequency than the frequency of the period setting signal and whose amplitude represents a voltage value that is lower than the high voltage HVDC value. A semiconductor switch accumulates electric charge on a capacitative element by means of the high voltage HVDC from the high voltage generator when the voltage value of the combined signal is lower than the set gate voltage value and generates an impulse voltage whose peak value is the value of the high voltage HVDC, by means of the electric charge that is discharged from the capacitative element when the voltage value of the combined signal exceeds the set gate voltage value.