This klystron (10) comprises an electron gun unit (A), a plurality of resonance cavities (14a) to (14j), a collector (15), and a plurality of drift tubes (16a) to (16k). The plurality of resonance cavities (14a) to (14j) include an input cavity (19), a plurality of intermediate cavities (21b) to (21i), and an output cavity (20), positioned sequentially along the direction in which electrons (11) travel from the electron gun unit (A). The plurality of intermediate cavities (21b) to (21i) include a plurality of second harmonics cavities (23d), (23g). The collector (15) traps the electrons (11) that have passed through the plurality of resonance cavities (14a) to (14j). The plurality of drift tubes (16a) to (16k) are provided between the electron gun unit (A) and the input cavity (19), between each of the plurality of resonance cavities (14a) to (14j), and between the output cavity (20) and the collector (15).