An ion beam irradiating apparatus comprises: a beam section monitor 14 measuring a beam current density distribution of an ion beam 4 in y-direction near an target 8; movable shield plate groups 18a, 18b opposite with each other and respectively having a plurality of movable shield plates 16 arranged in the y-direction to span an ion beam path at an upstream side of the position of the target, and are able to be moved in x-direction independently; shield plate driving devices 22a, 22b which can repeatedly drive the movable shield plates 16 composing the groups in the x direction in an independent manner; and a shield plate control device 24 which controls the shield plate driving devices 22a, 22b based on a measured data obtained from the monitor 14 to comparatively increase the ion beam amount of ion beam obstructed by the movable shield plates 16 at a position where the measured beam current density in the y-direction is comparative larger, so that the uniformity of the beam currentdensity distribution in the y-direction is enhanced.