PURPOSE: To vary the discharge characteristic through external control, by providing a control switching element for controlling the start/stop through a control signal in a self-oscillation circuit.

CONSTITUTION: A discharge energy accumulating capacitor 5 for receiving the output from D.C. high voltage power source 2 for boosting the voltage of a power source cell 1 through a choke coil 3 and a dide 4 is provided. A series circuit of the primary coil 6a of the firing coil 6 and the first switching element or thyristor 9 is connected across said capacitor 5. A self-oscillation feedback section comprised of a detection coil 23, capacitors 20, 14, resistor 19 and diode 18 is provided with respect to the gate of thyristor 9. Here a discharge control circuit comprised of second and third switching elements 16, 15, resistor 17, capacitor 13 and diode 12 is provided with respect to said gate to achive the purpose.