Abstract: According to one embodiment, a klystron includes an electron gun unit, a plurality of resonant cavities, a collector, and a plurality of drift tubes. The resonant cavities include an input cavity, a plurality of intermediate cavities, and an output cavity, positioned sequentially along the traveling direction of electrons from the electron gun unit. The intermediate cavities include a plurality of second harmonic cavities. The collector captures the electrons that have passed through the resonant cavities. The drift tubes are provided between the electron gun unit and the input cavity, between the resonant cavities, and between the output cavity and the collector.