A particle beam therapy system is disclosed which comprises: an accelerator (14) that is adapted to accelerate a charged particle beam； an irradiation apparatus that has a beam scanning apparatus (5a, 5b) for performing scanning with the charged particle beam and a first duct (4b, 6b) in which a beam outlet window (7) is provided at a more downstream position than the beam scanning apparatus (5a, 5b), and that is adapted to irradiate the charged particle beam onto an irradiation subject through the inside of the first duct (4b, 6b)； a beam transport apparatus (15) that has a second duct (1) and is adapted to transport, through the inside of the second duct (1), the charged particle beam exciting from the accelerator (14) to the irradiation apparatus； and a rotating gantry (19) in which part of the beam transport apparatus (15) and the irradiation apparatus are mounted, wherein the first duct (4b, 6b) is provided with duct extension/contraction means (20) that enable the beam outlet window (7) to travel in the axis direction of the charged particle beam； and in the case where the charged particle beam is irradiated onto the irradiation subject, the duct extension/contraction means (20) are extended, and in the case where the rotating gantry (19) is rotated, the duct extension/contraction means (20) are contracted.
