There is obtained a particle beam therapy system in which the beam size is reduced. There are provided an accelerator 14 that accelerates a charged particle beam； an irradiation apparatus that has a beam scanning apparatus 5a, 5b for performing scanning with the charged particle beam and irradiates the charged particle beam onto an irradiation subject； and a beam transport apparatus 15 that has a duct for ensuring a vacuum region or gas region that continues from the accelerator 14 to a beam outlet window 7 disposed at a more downstream position than the beam scanning apparatus 5a, 5b, and that transports the charged particle beam exiting from the accelerator 14 to the irradiation apparatus.
