To provide a gyrotron having the extremely improved frequency band characteristic of an output window, and wherein a temperature rise and thermal stress resulting from a high frequency loss are suppressed.

This gyrotron comprises an electron gun portion generating an electron beam, a resonant cavity portion disposed downstream of the electron gun portion, a collector portion arranged downstream of the resonant cavity portion, and an output portion 9 having a dielectric output window 8 passing an electromagnetic wave generated by the resonant cavity portion. Further a diamond layer 15 is formed on the surface of the output window 8. Thereby, expected performance of the gyrotron is attained.