An ion beam irradiating apparatus has a field emission electron source 10 which is disposed in a vicinity of a path of the ion beam 2, and which emits electrons 12. The field emission electron source 10 is placed in a direction along which an incident angle formed by the electrons 12 emitted from the electron source 10 and a direction parallel to the traveling direction of the ion beam 2 is in the range from −15 deg. to +45 deg. (an inward direction of the ion beam 2 is +, and an outward direction is −).