PURPOSE: To improve the conversion efficiency to a high frequency by controlling the collector voltage based on the judgement of the quality of the conversion efficiency to a high frequency and a detection signal obtained when an arc in a collector is detected by a photoelectric element.

CONSTITUTION: An efficiency judging means 16, a collector voltage determining means 17, and a voltage control means 18 are provided so that the conversion efficiency to a high frequency is calculated based on the current flowing through a slow-wave circuit 5 and a collector 6 and the amplified high-frequency electric power, an arc in the collector 6 is detected by a photoelectric element 10, and the collector voltage Vc for the most efficient operation is obtained. Thereby, a traveling wave tube which follows its operating condition to generate the lowest collector potential and has the high conversion efficiency to a high frequency can be obtained even if it has one collector.