A particle beam therapy system is provided with high irradiation flexibility that can reduce the amount of irradiation onto normal tissue. There are provided (i) a scanning electromagnet that performs scanning and outputting in such a way that a supplied charged particle beam is formed in a three-dimensional irradiation shape based on a treatment plan and (ii) deflection electromagnets that switch the orbits for the charged particle beam in such a way that the charged particle beam with which scanning and outputting are performed by the scanning electromagnet reaches an isocenter through a single beam orbit selected from a plurality of beam orbits established between the isocenter and the scanning electromagnet. The distance between the scanning electromagnet and the isocenter is made long.