To provide a circular accelerator capable of detecting the position of a circulating charged particle beam and a particle beam therapy system including the circular accelerator. A circular accelerator (39) configured to apply a first radio frequency to a circling charged particle beam to accelerate the charged particle beam, the circular accelerator including: an electrode (12), (13) configured to apply the first radio frequency to the charged particle beam; and a beam position monitor (16) provided in the electrode and configured to detect a position of the charged particle beam.