PURPOSE: To prevent the leakage of the TE11 mode wave propagating toward the axle by providing a circular tuner movable in the extending direction within the resonance cavity and then setting the radius of both the tuner and the probing hole on the inner wall of the cavity so as to satisfy the prescribed formula.

CONSTITUTION: An arc-shaped probing hole is formed at part of upper and lower walls 3 and 4 of the cavity extending vertically to electron beam 2, thus forming arc- shaped cavity upper and lower walls 5 and 6. And resonance frequency variable and mobile tuner 7 moves along wall 5 and 6. Tuner 7 features a circle cross-section and a choke structure of 1/4 wavelength at the tip part. Now, radius R1 of tip part 8 of tuner 7 and radius R2 of the probing hole are selected so that formula λc π(R1+R2)<λ may be satsfied, where λc is the wavelength of the cut-off frequency of the TE11 mode wave and λ is the wavelength of the free space respectively. As a result, a cavity resonator which can prevent effectively the leakage of the TE11 mode wave is obtained.