In a peniotron, a hollow electron beam is generated from a cathode gun assembly and a DC magnetic field is applied to the electron beam from solenoid coils. Thus, each electron of the electron beam is gyrated into a resonant cavity and into propagating waveguide sections which are maintained in auto-resonant conditions so that the electrons interact with an electromagnetic waves of TE mode not only in the resonant cavity section but also in a waveguide section. Accordingly, the electromagnetic wave is oscillated in the resonant waveguide section and amplified in the propagating waveguide section in such a manner that the level of the electromagnetic wave in the resonant cavity section is far less than the output power from said propagating waveguide.