PURPOSE: To enable electromagnetic wave beams of high frequency and high output power applicable to plasma heating of a nuclear fusion apparatus, to be generated to be generated efficiency by providing a magnetic field changing means in the vicinity of a resonator.

CONSTITUTION: In a resonator 6, a ferromagnetic ring 12 is provided between a magnetic coil 4 and a mirror 8 so that it can move in a space 14 formed in the wall of a vessel 13. The movement of this ring 12 is controlled by the hydraulic pressure of a hydraulic device 15 and is capable of changing the magnetic field of the resonator 6. After oscillating electromagnetic wave 5 of a desired frequency by making electron beam 1 incident inside resonator 6, magnetic field 5 in the resonator 6 is changed to change the cyclotron frequency of electorons and shift the frequency difference Δω, in order to optimize the efficiency. This change of magnetic field is not accompanied by the oscillation of electromagnetic wave of another frequency. By this configuration, electromagnetic wave of the desired frequency can be generated with high efficiency.