Disclosed is an ion beam irradiation apparatus including: a plurality of plate-like grid electrodes arranged in a beam irradiation direction so as to overlap each other and each having a plurality of apertures; a power supply unit that applies a voltage to each of the grid electrodes; and a controller that controls the voltage applied to each of the grid electrodes by the power supply unit. The plurality of grid electrodes include first to fourth grid electrodes. Central axes of apertures of the first grid electrode and apertures of the second grid electrode are coaxial along the beam irradiation direction, and a central axis of apertures of the third grid electrode is offset in a direction orthogonal to the beam irradiation direction with respect to the central axes of the apertures of the first grid electrode and the second grid electrode.