PURPOSE: To reliably generate discharge without the particular increase in a pulse output voltage of a discharge power source, by a method wherein a discharge trigger electrode is situated in a hollow part of an anode, and a potential, being lower than that of a cathode, is applied to the discharge trigger electrode.

CONSTITUTION: A discharge trigger electrode 13, supported by a retainer, is insulated from a cathode 3, and extends into the hollow part of the anode 1 after piercing the perforation in the cathode. One end of an output of a trigger electrode power source 24 is connected to a negative high-pressure output, and the other end of the output is applied to the discharge trigger electrode 13. A potential, being lower than that of the cathode 3, is applied to the discharge trigger electrode 13, and this shortens a time which has elapsed until a discharge is generated after a voltage between the electrodes for discharge and a magnetic field are applied.