PURPOSE: To enable accurate measurement of respective current and voltage values of a microwave oscillator, and effective protection of the microwave oscillator from abnormal condition by providing a means to isolate the casing of the microwave oscillator from earth potential.

CONSTITUTION: An insulator 23 is placed between a flange 20 connecting a gyrotron 10 with magnet container 21 and the magnet container 21, and insulating bolts are used to fix the flange 20 to the magnet container 21. The casing 14 of the gyrotron 10 can be thus isolated from earth potential of the magnet holder 21 and an oil tank 24. On the other hand, a main collector circuit conductor 7 is grounded directly. Thus, the main collector circuit conductor 7 is fixed at earth potential and the common line of a measurement system is prevented from potential floating.