MAGNETRON

PROBLEM TO BE SOLVED: To restrain radiation of a plurality of kinds of spurious electromagnetic waves such as many harmonics over a wide frequencies band by forming a plurality of choke structures of one metallic cylinder. SOLUTION: One end of an antenna lead 17 is electrically connected to one of vanes 2, and the other end thereof is fixed to a sealing portion of an exhaust tube 16 through the center of an insulating cylinder 15. The lead 17 is provided with wide and narrow portions which are different in vertical and horizontal. lengths in cross section. A metallic cylinder 13 on an output side is connected directly to an anode cylinder 1. Microwave electric power oscillated in an anode travels on the lead 17 serving as a coaxial line extending on the center axis of the metallic cylinder 13, to be thus emitted through the tip thereof. Another metallic cylinder 21 forming a choke structure is fixed to the inside of the metallic cylinder 13 having lengths, which are different from position to position in a circumferential direction and are set to about 1/4 of a wavelength of an electromagnetic wave for restraining radiation, thereby forming a step at the tip thereof. An electric field is firmly coupled to the lead 17 at the tip of each step of the stepped cylinder 21.