PURPOSE: To obtain a multi-cavity klystron that can be amplified over a wide frequency band by providing more than two high frequency circuit sections with different fequency bands and allowing these high frequency circuit sections to have their own independent input and output circuits.

CONSTITUTION: An electron beam emitted from an electron gun is focussed by an electron beam focussing device and passes through the first high frequency circuit section 2. It also passes through the second high frequency circuit section 2' that can be tuned at a different frequency band with that of the first high frequency circuit section and is collected by a collector. As a result, when a multi-cavity klystron is operated at any frequency within the fist frequency band 5.8 to 6.4GHz, a signal can be input to the first high frequency circuit section only. Besides, when the klystron is operated at any frequency within the second frequency bands 6.4 to 7.0 GHz, an input circuit can be switched depending upon each frequency band so that the signal can be input to the second high frequency circuit only.