To provide a gyrotron having a mode converter that generates effectively a millimeter wave beam while suppressing parasitic mode oscillation.

The gyrotron comprises: an electron gun which has an anode and a cathode and discharges an electron beam, a cavity resonator which generates an electromagnetic wave by accepting and using electron beam excitation, a mode converter, which is cylindrical, has a focusing mirror formed along the inner wall, receives the electromagnetic wave, and makes the electromagnetic wave condense into a beam with the condenser mirror, and an output window form which the electromagnetic wave condensed into the beam is output, where the inner wall average radius of the mode converter increases monotonically from the inlet to the outlet of the mode converter, and the axial period of deformation forming the condenser mirror increases monotonically from the inlet to the outlet of the mode converter.