PURPOSE: To reduce dispersion of the value of out-off voltage by disposing one or more ceramic spacers between a plurality of cathode structures and the G1, G2 electrodes, fixing them by welding with a seal glass and forming integrated three-electroce structure.

CONSTITUTION: A ceramic spacer 22 having the surface ground with high accuracy and uniform plate thickness is placed between the G1 electrode 4', and G2 electrode 5'. At the lower surface of G1 electrode 4', the ceramic spacer 23 for holding the electrode is inserted and the four poles 15 are provided in line with the internal diameter to fix by welding the support 16 while holds the cathode supporting member 12 and the heater 13. Each part is covered with a case 24 welded to the inner side surface of G2 electrode and its internal core is formed in an integrated structure fixed by welding with the seal glass 25. Thereby, the gap between G1 and G2 is stabilized to a constant value and thereby a gap between K and G1 can also be set with high accuracy, thus preventing dispersion of the value of cut-off voltage.