The present invention provides a multi-beam klystron apparatus (11). In the above-described multi-beam klystron apparatus, the magnetic field generating element (52) of the electron-gun-unit-side magnetic field generating unit (41) is disposed around the electron gun unit (18), and a plurality of magnetic gaps (54, 55) are provided in the inner peripheral surface of the magnetic pole (53), which covers the magnetic field generating element, in the direction of travel of the electron beams. Therefore, lines of magnetic force, which are parallel to the center axis of the radio-frequency interaction unit (19), can be generated. Thus, even the electron beam, which is generated from the location apart from the center axis of the electron gun unit, can be guided to the radio-frequency interaction unit in the same manner as in the location at the canter axis of the electron gun unit.