PURPOSE: To promote the efficiency of the input/output conversion of a klystron device by arranging a magnetic flux diffusing member between the output drum and a collector portion side magnetic pole plate, and lowering suddenly the distribution of the density of magnetic flux from the vicinity of the drum gap of an output drum.

CONSTITUTION: A magnetic flux diffusing member 50 is arranged concentrically with the axis C of a drift tube 37 on the periphery of the drift tube 37 positioning between an output drum 36 and a collector portion 38 side magnetic pole 40. The member 50 is made by binding a magnetic cylinder 51 on the periphery of a reinforcing cylinder 62. Then, the distribution of the density of magnetic flux on an electron beam passage lowers suddenly from the vicinity of the gap (g) of the output drum, and decreases successively yo the vicinity of the central hole of the magnetic pole 40. Accordingly, slowed electrons after passing the gap (g) are curved to the radial direcrion and are caught on the inner wall surface of the drift tube 37, the drop of electric potential in the tube 37 is prevented, and the generation of a retrograde electron is restrained. Thereby, the efficiency of input/output conversion can be promoted by a simple structure.