PURPOSE: To restrain the synchronous oscillation of plural modes by providing plural emission belts whose radii are different, at a cathode.

CONSTITUTION: The 1st and 2nd emission belts 11, 12 which are provided at a cone base shaped cathode 1 and emit electrodes, are made to be circular annular shapes whose radiuses are mutually different, and a portion 13 which does not emit electrons, is arranged between them. Electrons emitted from these emission belts become the 1st and 2nd electron beams 14, 15 which are of a hollow shape and whose radii are mutually different. For example, in the case of a TE03 mode being oscillated, as shown by the distribution 16 of the strength of the coupling of electron beams and the electromagnetic waves of the TE03 mode in an oscillating cavity 6, the radii of emission belts and the strength of a magnetic field at the cathode 1 or the like are previously set so that the 1st and 2nd electron beams may pass positions where their coupling to an objective mode becomes respectively maximum in the oscillating cavity, and then both beams 14, 15 have strong coupling to the TE03 mode, and obtain strong oscillation, and also an adjoining TE52 mode is not oscillated, and competition is restrained.