PURPOSE: To improve the reproducibility and to avoid the effect of thermal drift or the like by providing a tuning element drive section changing a resonance frequency of a cavity resonator and a mechanism sensing the resonance frequency of the cavity resonator to the mechanism.

CONSTITUTION: The resonance frequency of a cavity resonator 1 is varied by driving a motor 5 being a component of a tuning element drive mechanism 4 connecting to a tuning rod 3 whose tip is fitted to a tuning element 2. In this case, the resonance frequency is detected by a frequency detection element 6 and a frequency detector 7 fitted to the cavity resonator 1. When the band characteristic of a multi-cavity klystron is taken at a center frequency, it is possible to preset the band characteristic by storing the resonance frequency of each cavity to a resonance frequency comparison device 8. Furthermore, when the resonance frequency is changed due to thermal drift or the like by a feedback device 9, the preset resonance frequency is automatically restored and failure due to thermal drift or the like is solved.