PURPOSE: To stop the oscillation operation of the inverter to prevent the damage of circuit elements when high-voltage discharge pulses are generated continuously in the secondary-side circuit, by connecting a resistance in parallel to the capacitor to constitute a time constant circuit for discharge.

CONSTITUTION: When the generation interval of high-voltage discharge pulses is larger than the time constant determined by capacitor 23 and resistance 29, switching transistor 24 is not turned on, and therefore, the oscillation operation continues still. When the generation interval of high-voltage discharge pulses is smaller than the time constant, the charging quantity of capacitor 23 is increased gradually; and if the voltage dependent upon this increment reaches the sum of the breakdown voltage of Zener diode 30 and the voltage between the base and the emitter which makes switching trasistor 24 turned on, charged electric charge of capacitor 23 forms the base current of transistor 24 at a time. Therefore, transistor is turned on from the turning-on state to stop oscillation of oscillating circuit 31, so that damage of circuit elements can be prevented.