In an ion beam irradiating apparatus, a specified ion beam is first deflected in a deflection direction perpendicular to an ion beam orbit by an ion beam deflector. The deflected ion beam is neutralized by a thermoelectron beam emitted from a filament of an ion neutralizer. An electrode is employed to control the supply of the thermoelectron beam to the deflected ion beam. Both the filament and control electrode elongated along the deflection direction surround the deflected ion beam traveled along the ion beam orbit.