PURPOSE: To improve the voltage withstandability of an electron source under high voltage and to facilitate the handling, by providing an outer casing in which an electrical insulation fluid can be filled at the high voltage section of a high power electron tube.

CONSTITUTION: An acceleration electrode 2, cathode electrode 3 are connected through an alumina field 4 for insulation as an electron source at the high frequency circuit 1 which is in the ground potential. While an outer casing 6 made of an insulator is supported as said circuit 1. The heater cable 8, heater cathode cable 9 and the anode cable 10 applied with high potential are taken out to the outside of the casing 6 through a bush 7 for the purpose to prevent the leakage of the electrical insulation fluid in the casing 6.