PURPOSE: To enable the output of an electromagnetic wave beam having a Gaussian electromagnetic field distribution of an excellent high frequency trans mission characteristic by arranging a mode converter, a radiator to radiate a high frequency electric power to the outside of a gyrotron at the downstream of the mode converter through an output window section and a reflecting plate.

CONSTITUTION: An electron beam emitted from an electron gun 41 enters a resonating cavity section 44 through an electron beam inducing section 43 by being guided in a strong magnetic field in the axial direction, and the energy of the electron beam is converted into an electromagnetic wave energy. After this, the electron passes through a mode converter 45 and a radiator 46, and is absorbed in a collector 48. A corridor mode huge electric power high frequency generated in the resonating cavity section is mode-converted from a corridor mode into a mode symmetrical to a circular axis by means of a Thumn type converter 45, and is radiated toward a reflecting plate 47 with the radiator 46 being cut stepwise or obliquely, and outputted outside as a Gaussian electromagnetic wave through an outputting ceramic window 50.