PURPOSE: To make magnets compact, reduce the irregularity of mounting of the magnets and heighten the accuracy of a magnetic circuit, by providing a gas evacuation cylinder at one end of an anode cylinder, at which an output antenna portion would be conventionally disposed.

CONSTITUTION: One end of an antenna lead 5 is coupled to a vein 2 through the coupling hole 6 of the circumferential portion of an anode cylinder so that microwave energy generated in the anode cylinder is transmitted to an output antenna portion 7. A cathode support 8 has electric power feed terminals 9, 10. A gas evacuation cylinder 12 is provided at the central portion of a sealing plate 11 which seals the end face of the anode cylinder. The cylinder 12 is sealed and cut off to a prescribed length at the end of gas evacuation of the interior of the anode cylinder. According to this constitution, the inside diameters of magnets can be decreased to make the magnets compact, and the irregularity of mounting of the magnets can be absorbed to form a magnetic circuit of high accuracy.