An objective of this invention is to accurately calculate beam irradiation position and perform highly accurate beam irradiation even when positions of a scanning magnet and a position monitor are relatively changed. A particle beam irradiation apparatus includes a position monitor (9) detecting a passing position of a charged particle beam (2), and an irradiation control device (23) calculating a position motor distance information which is the distance from an irradiation surface (31) of an irradiation object (25) to the position monitor (9) in a beam axis direction, from a position information of the position monitor (9) in the beam axis direction, and calculating a beam irradiation position (P) of the irradiation object (25), so as to control the irradiation of the beam (2). The irradiation control device (23) has a position calculation device (30) for calculating the irradiation position (P) which is represented based on a distance from an intersection between the beam axis (27) and the irradiation surface (31), based on a beam position (Pm) detected by the position monitor (9), a scanning origin distance information from the irradiation surface (31) of the irradiation object (25) to a scanning origin (1x, 1y) of the beam (2) at scanning magnets (5x, 5y), and the position monitor distance information.
