To provide a gyrotron that simply, safely, and accurately removes an undesired electromagnetic wave, and secures stable and high efficient operation.

This gyrotron comprises an electron gun portion generating a hollow electron beam, a tapered beam tunnel portion 14 arranged downstream of the electron gun portion, and having an inner wall gradually lessening in its diameter, a resonant cavity portion 3 disposed downstream of the beam tunnel portion, and a collector portion arranged downstream of the resonant cavity portion. At least one opening 15 is formed in the conductor wall of the beam tunnel portion 14 passing the undesired electromagnetic wave of the beam tunnel portion 14. Expected performance of the gyrotron is attained by such constitution.