Accelerator and particle therapy system

An accelerator that accelerates an ion beam while circulating the ion beam by a main magnetic field and an accelerating radiofrequency electric field, the accelerator comprising:

• a main magnetic field generation device including a plurality of magnetic poles arranged to face each other and exciting the main magnetic field in a space sandwiched between the magnetic poles;
• an extraction channel through which the ion beam is taken out;
• a displacement unit configured to displace an ion beam circulating in a main magnetic field region where the main magnetic field is excited to an outside of the main magnetic field region;
• and
• a disturbance magnetic field region provided on an outer peripheral portion of the main magnetic field region, the disturbance magnetic field region being configured to excite a magnetic field that disturbs the ion beam displaced to the outside of the main magnetic field region and guides the ion beam to the extraction channel,
• wherein the disturbance magnetic field region includes
• a first region in which the strength of the magnetic field decreases toward the outside,
• a second region in which the strength of the magnetic field increases toward the outside, and
• a third region in which the strength of the magnetic field is larger than the strength of the magnetic field in the first region and smaller than the strength of the magnetic field in the second region.