PURPOSE: To reduce noise and enhance stability of oscillation by providing the third electrode plate which is protruded into the electron working space extending toward the anode through gaps of cathode equally divided into several sections in the circumferencial direction and is in form of a plate larger than the external diameter of cathode.

CONSTITUTION: The electron emitting cathodes 70 are coaxially disposed and these cathodes constitute a cavity resonator in combination with the anode vain 26 which is also disposed coaxially. The third electrode plates 71 are arranged near the cathodes 70. These electrode 71 consist of plate-like bodies protruded in the axial direction to the side of electron working space 39 from gaps 72 between the cathodes 70 and are a little larger in outer diameter than the cathodes 70. The electrodes 71 are also integratingly supported by a cylindrical electrode supporting structure 73 located between the cathodes 70 and internal cathode support 72. Thus, stability of oscillation can be maintained without interference on scattering of electrons in the axial direction and noise can also be lowered.