PURPOSE: To easily obtain a high power klystron having a good delay characteristic and a stabilized output by suppressing a thermal drift by working a resonant cavity for its dimension by means of cutting, etc., in the intermediate process of the assembling of said cavity.

CONSTITUTION: The dimensional accuracy of a resonant cavity is considered so as to provide the higher resonance frequency. Here, the first and second recess part 24 and 26 are subjected to cutting work to provide cutting parts 29, 30 such that the resonance frequency is measured in a mechanically assembled state, and thereby the required resonance frequency is provided before the first and second high frequency circuit parts 23, 28 are assembled by means of brazing, etc. By preforming cutting work such that required resonance frequency is obtained in this way in the intermediate process of the assembling, required resonance frequency can be easily obtained without seriously restricting the accuracy of the dimension of parts to be worked.