PURPOSE: To heat the thermal cathode and an auxiliary electrode while using only a DC power supply for maintaining the required temperatures thereof, by preparing a metal shell having many uniformly made holes or a metal reticular thermal emitter between the thermal cathode and the auxiliary electrode.

CONSTITUTION: A thermal emitter 7 having many uniformly made holes is prepared between a thermal cathode 5 and an auxiliary electrode 6. When a tungsten filament 8 is preheated and a DC heating power supply 11 is connected to the filament as shown by a dotted line, the auxilliary electrode 6 is heated by the electrons emitted from the tungsten filament 8. When the temperature of the auxiliary electrode 6 reaches the prescribed value, the DC heating power supply 11 is changed over as shown by a real line. Then, the heat emitter 7 and the thermal cathode 5 are heated by the electrons emitted from the auxiliary electrode 6 so that the auxiliary electrode 6 is maintained at the required temperature in turn due to said radiant heat. In this way, sufficient amount of electrons are supplied into the main discharge room 4 in order to perform arc discharge by using a discharge power supply 12 while heating the thermal cathode 5 and maintaining its temperature.