PURPOSE: To enlarge electric field intensity on an electrode surface and to make releasing of heat electrons from an electrode easy by arranging both of a lump- shaped heat negative pole and a filament heat negative pole in a discharging chamber of ion source and by providing with a means to ignite gas discharging in the chamber.

CONSTITUTION: A lump-shaped heat negative pole 5 is heated by electronic beam released from a heated tungsten filament 6. Further, one or a plurality of filament heat negative pole 7 are arranged in a discharging chamber, and they are electrically connected to a discharging positive pole wall 1 through a switch and a condenser 10. Thereby, heat electrons are released from the heated negative pole 7 by condenser discharging, and gas discharging in the form of pulse is performed. As a result, plasma is generated on the surface of the lump-shaped electrode 5, and thereby electric field intensity on the electrode 5 surface is enlarged, a large quantity of heat electrons are released from the electrode 5, and gas discharging can be maintained even after there is no electric charge in the condenser 10.