Particle beam irradiation apparatus and particle beam therapy system

A particle beam irradiation apparatus that irradiates a charged particle beam accelerated by an accelerator onto an irradiation subject, the particle beam irradiation apparatus comprising:

• a vacuum duct that forms a vacuum region through which the charged particle beam passes;
• a vacuum window that includes a nonmetallic window plate, and is provided at the downstream side of the vacuum duct and through which the charged particle beam is launched from the vacuum region;
• a scanning electromagnet that scans the charged particle beam in a direction that is perpendicular to a beam axis;
• a monitoring apparatus including a position monitor that detects a passing position of the charged particle beam and a beam size thereof;
• a low-scattering gas filling chamber that covers the vacuum window and at the downstream side of which, the monitoring apparatus is disposed;
• and
• an irradiation management apparatus that controls irradiation of the charged particle beam,
• wherein 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
• wherein when the charged particle beam is irradiated, the low-scattering gas filling chamber is filled with a low-scattering gas that scatters the charged particle beam less than air does.