To provide an electron tube capable of effectively discharging a gas generated from an electron gun side and a collector side without having a complicated structure and suitably restraining electric discharge in the tube and variation of oscillation conditions.

In a gyrotron having an electron gun 1 for generating an electron beam, a magnet 25 for preparing a magnetic field, a cavity 6 for generating an electromagnetic wave, and a collector 11 for collecting electron beams, a cooling layer 20 is arranged on an outer periphery of the cavity 6, a high-vacuum layer 21 is arranged on an outer periphery of the cooling layer 20, and the high-vacuum layer 21 is space-combined with a collector 11 side via an exhaust port 22, and is space-combined with an electron gun 1 side via an exhaust bypass 23. Thus, deterioration of the vacuum degree is prevented since residual gas at the electron gun 1 side is exhausted outside without passing through the cavity 6. Further, overheating of tapered waveguides 5a, 5b and the cavity 6 as well as the collector 11 can be prevented by the cooling layer 20.