An electron beam generated by an electron gun is oscillated in a cavity resonator and output as a millimeter electromagnetic wave. The output electromagnetic wave is transmitted to a cylindrical mode converter. The inner wall surface of this mode converter has a plurality of sets of ridges and grooves spirally formed at equal pitches so as to gradually change the degree of corrugation in the transmission direction of the electromagnetic wave. The mode converter separates the input electromagnetic wave from the cavity resonator into a plurality of electromagnetic waves having the same power distribution. The output electromagnetic waves from the mode converter are transformed into wave beams by mirror systems and output to the outside of a gyrotron from output windows provided in a one-to-one correspondence with these electromagnetic waves.