PURPOSE: To absorb and dampen an electromagnetic wave leaking from the gap of a waveguide separation part with an electric wave absorbing body and preventing the electromagnetic wave from reaching an exhaust pump by providing the electric wave absorbing body in an exhaust pipe leading to an exhaust pump.

CONSTITUTION: An electron beam 11 emitted from the electron gun part 1 of a gyrotron is made to give a turning motion having a cyclotron frequency due to an electric field generated by an external electromagnet 7 and another electric field applied to a space between an anode and a cathode. The aforesaid turning motion is injected to a resonance cavity 3 while being increased with an adiabatic compression effect of a mirror electric field, and electronic energy is thereby converted to high-frequency energy. Furthermore, the beam 11 is converted to heat through collision with the internal wall surface of a collector 5. Gases generated at the time of the collision are exhausted with an ion pump from the gap of a waveguide separation part 10 via a vacuum chamber 14 and an exhaust pipe 18. An electric wave absorbing body 31 positioned in the exhaust pipe 18 absorbs and dampens the electromagnetic wave leaking from the gap of the separation part 10, thereby preventing the wave from reaching the ion pump.