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.
