The objective is to irradiate a small-size charged particle beam onto an irradiation subject even when the energy thereof is low. A particle beam irradiation apparatus (58) according to the present invention is provided with a vacuum duct (6, 7) that forms a vacuum region through which the charged particle beam (1) passes, a vacuum window (8) through which the charged particle beam (1) is launched from the vacuum region, a scanning electromagnet (2, 3) that scans the charged particle beam (1) in a direction that is perpendicular to a beam axis； a monitoring apparatus (67) including a position monitor (5) that detects the passing position of a charged particle beam (1) and the beam size thereof, a low-scattering gas filling chamber (39) that covers the vacuum window (8) and at the downstream side of which, the monitoring apparatus (67) is disposed, and an irradiation management apparatus (20) that controls irradiation of the charged particle beam (1)； the particle beam irradiation apparatus (58) is characterized in that the low-scattering gas filling chamber (39) is changeably disposed in such a manner that the beam-axis-direction positional relationship between the monitoring apparatus (67) and the vacuum window (8) is a desired one and in that when the charged particle beam (1) is irradiated, the low-scattering gas filling chamber (39) is filled with a low-scattering gas that scatters the charged particle beam (1) less than air does.
