PURPOSE: To control an inverse electron and obtain the effect of energy saving such as power saving by using at least one small inner diameter drift tube in intermediate cavity drift tubes.

CONSTITUTION: At least one small inner diameter drift tube 10 is provided in intermediate cavity drift tubes. Besides, the inner diameter d of this drift tube 10 is set to less than 0.9 of the inner diameter a of another drift tube 9 and set to 1.1 times the outer diameter b of an electron beam 2. However, in general, a multi-cavity klystron is designed so that the electron beam diameter can be within 0.8 times the inner diameter of the drift tube 9. That is, the drift tube 10 is configured so as to have a relationship of 1.1b≤d≤0.9a. By setting the inner diameter of the drift tube 10 to d≥1.1b, the collision of the electron beam 2 which advances toward the collector side becomes small. Besides, by configurating the drift tube 10 so as to have a relationship of d≤0.9a, the anode current can be reduced and inverse electron can be caught from the collector.