PURPOSE: To set electron beams at the position of the largest oscillation efficiency without their collision against the circuit wall even in the operation of a high order mode and a large current operation by planting an electrode on the inner surface of a beam tunnel through an insulator.

CONSTITUTION: On the inner surface of a beam tunnel 14 to lead the electron beams injected by an electron gun 1 to a high-frequency resonance circuit 3, an electrode 15 is planted through an insulator 16. As a result, when the central axis of the electron beams 2 is slipped from the tube axis, a current to indicate the slippage flows to the electrode 15. Consequently, the passing position of the electron beams 2 can be set at the position to make the oscillation efficiency maximum without collision of electron beams 2 with the circuit wall, by deciding the direction of the slippage from the flow of the current.