Accelerator and particle therapy apparatus

A particle therapy apparatus comprising:

• an accelerator that accelerates an ion beam while causing the ion beam to circulate by a main magnetic field and an acceleration radiofrequency electric field;
• and
• an irradiation device that performs irradiation with the ion beam extracted from the accelerator,
• wherein the accelerator includes
• a main magnetic field generation device that applies the main magnetic field to a space between a plurality of main magnetic poles arranged to face each other,
• a beam displacement device that causes the ion beam circulating in a main magnetic field region to which the main magnetic field is applied to be displaced to outside of the main magnetic field region,
• an extraction channel magnetic field generation device that generates an extraction channel magnetic field to extract the ion beam that has been moved to the outside, and
• a canceling magnetic field generation device that is provided to be closer to an inner peripheral side than the extraction channel magnetic field generation device and generates a canceling magnetic field with a polarity opposite to a polarity of the extraction channel magnetic field.