PURPOSE: To allow operation with a relatively weak magnetic field and a high microwave frequency by forming a preset angle between the propagation direc tion of microwave which is propagated in a slow-wave circuit and the running direction of electron beam in arrangement.

CONSTITUTION: A preset angle α is formed between the propagation direction of TM-mode microwaves 22 which are propagated in a slow-wave circuit 21 and the running direction of electron beam 23 in arrangement. A magnetic field B0 is applied to the direction perpendicular to the electron beam. In this case, the relationship V0=E0/B0 is satisfied, where V0 is the velocity of the electron beam, B0 is the intensity of a magnetic field and E0 is an excited high frequency electric field. The relationships rp.V0.B0=E0 and W>WC also hold under the condition of rp=[1-(VP/c2]-1/2, where VP is the phase velocity of electromagnetic wave, c is the velocity of light, W is the angular frequency of the electromagnetic waves and WC is the angular frequency of cyclotron determined by the magnetic field. Operation is allowed with a relatively weak magnetic field but a high microwave frequency, resulting in higher efficiency.