Particle beam therapy device and irradiation field forming method

To provide a particle beam therapy device that expands an irradiation field while avoiding an increase in size of a scanning unit or an irradiation device including the scanning unit. A shift unit 36 is provided downstream of a scanning unit 34. The shift unit 36 deflects a carbon beam as a particle beam to shift the irradiation field, thereby forming an expanded irradiation field. The shift unit 36 includes a first shift electromagnet 42 that shifts the irradiation field in a Y direction and a second shift electromagnet 44 that shifts the irradiation field in an X direction. The scanning unit is dynamically controlled, and the shift unit 36 is statically controlled.