PURPOSE: To prevent an oscillation event caused by reverse electron flow and perform stable operation by arranging a magnet or ferromagnetic body at the outside of a tube at least in a single location on the way to an electron beam path so as to disturb a focussing magnetic field.

CONSTITUTION: A rectilinear propagation klystron device consists of a tube main body in which an electron gun section 1, electron operation section 2, and collector section 3 are connected and a main magnetic coil 4 for the main magnetic field which is a focussing magnetic field device and arranged by connecting a number of cavity resonators 7 to 11 to the electron operation section 2 using a drift tube 12. Besides, the main magnetic field and a reverse auxiliary magnet 14 are arranged outside the drift tube 12 and between the third and fourth cavity resonators 9 and 10. Thus by locally changing the magnetic line 16 within the drift tube 12 using the auxiliary magnet 14, a reverse electron flow is expanded along the magnetic line 16, caught by the drift tubr 12, and extinguished. As a result, an oscillation event caused by the reverse electron flow can fully be prevented.