The invention provides an ion implantation apparatus which can be widely applied. The ion implantation apparatus includes a scanning unit (1000) which comprises a scanning electrode device (400) applying a deflecting electric field to an ion beam (B) along a reference trajectory (Z),the horizontally scanning ion beam (B) being orthometric with the reference trajectory (Z), and a downstream electrode device (500) disposed downstream of the scanning electrode device (400), and provided with an opening portion used for pass of the ion beam (B). The scanning electrode device (400) includes a pair of scanning electrodes (410R, 410L) disposed to face each other in the horizontal direction with the reference trajectory (Z) interposed therebetween. The downstream electrode device (500) includes an electrode body configured such that, with respect to an opening width in a vertical direction perpendicular to both the reference trajectory (Z) and the horizontal direction and/or an opening thickness in a direction along the reference trajectory, the opening width and/or the opening thickness in a central portion in which the reference trajectory (Z) is disposed is different from the opening width and/or the opening thickness in the vicinity of a position facing the downstream end (422) of the scanning electrode (410R, 410L).