PURPOSE: To enable a large power operation by forming a collector in a taper form making the diameter larger gradually to the lower stream side of the electron beams, and providing plural nozzles to spray a coolant to the outer wall of the collector.

CONSTITUTION: The electron beams 13 which have finished mutual reactions in a resonance cavity 3 are guided by an external magnetic field, struck to the inner wall of a collector 7, and converted to heat. Since the collector 7 is formed in a taper making the diameter larger gradually toward the lower stream side of the electron beams, the electron beams 13 can be caught along the wider scope than in the conventional cylindrical collector. As a result, the thermal load in the collector 7 can be reduced. Plural nozzles 10 to spray a coolant 9 are provided to the outer periphery of a boiler jacket 8, and a system to cool the collector 7 by striking and circulating the coolant is provided. Consequently, the cooling efficiency in the collector 7 is increased, and a gyrotron to enable a large power output operation can be provided.