A particle beam irradiation apparatus according to the present invention is provided with a vacuum duct that forms a vacuum region through which the charged particle beam passes, a vacuum window through which the charged particle beam is launched from the vacuum region, a scanning electromagnet that scans the charged particle beam; a monitoring apparatus including a position monitor that detects the passing position of a charged particle beam and the beam size thereof, a low-scattering gas filling chamber including the monitoring apparatus, and an irradiation management apparatus that controls irradiation of the charged particle beam; the particle beam irradiation apparatus is characterized in that the low-scattering gas filling chamber is changeably disposed in such a manner that the beam-axis-direction positional relationship between the monitoring apparatus and the vacuum window is a desired one and in that the low-scattering gas filling chamber is filled with a low-scattering gas.