An ion beam irradiating apparatus has: a beam profile monitor 14 which measures a beam current density distribution in y direction of an ion beam 4 in the vicinity of a target 8; movable shielding plate groups 18a, 18b respectively having plural movable shielding plates 16 which are arranged in the y direction so as to be opposed to each other across an ion beam path on an upstream side of the position of the target, the movable shielding plates being mutually independently movable in x direction; shielding-plate driving devices 22a, 22b which reciprocally drive the movable shielding plates 16 constituting the groups, in the x direction in a mutually independent manner; and a shielding-plate controlling device 24 which, on the basis of measurement information obtained by the monitor 14, controls the shielding-plate driving devices 22a, 22b to relatively increase an amount of blocking the ion beam 4 by the opposed movable shielding plates 16 which correspond to a position where a measured y-direction beam current density is relatively large, thereby uniformity of the beam current density distribution in the y direction.