PURPOSE: To allow the cavity part of a lead-out electrode body to be used is the space of earth potential by leading a high frequency current into a cavity resonator from high frequency power source to boost its voltage, and then leading it out from one opposed plate forming a parallel gap to be a high voltage generating part, in the electrically insulated state from the other substrate.

CONSTITUTION: In the case of using high frequency region power, the high frequency power led into a cavity resonator 1 from high frequency power source flows on the conductor wall inner surface so as to be resonated and boosted in a parallel gap G utilizing the principle of a current flowing only the surface or surface layer part of a conductor by skin effect. The high frequency power then flows on the outer face of a lead-out electrode body 12 extendedly provided at the center part of an opposed plate 8 forming the parallel gap G and reaches the tip of the lead-out electrode body 12 so as to apply the high frequency voltage-current to a wire electrode 32 stretched in electrical contact between the tips of the lead-out electrode body 12. At this time, the cavity part 13 of the lead-out electrode 12 can be used as the space of earth potential.