A particle therapy system (1; 1B; 1D) comprising: a circular accelerator for accelerating a charged particle beam; a charged particle beam irradiation apparatus for irradiating the charged particle beam extracted from said circular accelerator to an irradiation target; and an energy determination device for determining the energy of the charged particle beam, wherein said circular accelerator includes a cavity for applying a radio frequency wave to accelerate the charged particle beam, characterised in that the particle therapy system (1; 1B; 1D) further comprises: a frequency counter for measuring the frequency (Fmes) of said radio frequency wave; and a radial beam position measuring device for measuring the orbit position (Rmes) of the charged particle beam, and the energy determination device is configured: to measure whether or not the energy of the charged particle beam orbiting within said circular accelerator differs from a predetermined energy value after the end of acceleration of the charged particle beam, and to determine that the energy of the charged particle beam orbiting after the end of the acceleration is normal, when the frequency (Fmes) of said radio frequency wave after the end of the acceleration is within a first allowable range (Ferr) and the orbit position (Rmes) of the orbiting charged particle beam after the end of the acceleration is within a second allowable range (Rerr). a circular accelerator for accelerating a charged particle beam; a charged particle beam irradiation apparatus for irradiating the charged particle beam extracted from said circular accelerator to an irradiation target; and an energy determination device for determining the energy of the charged particle beam, wherein said circular accelerator includes a cavity for applying a radio frequency wave to accelerate the charged particle beam, characterised in that the particle therapy system (1; 1B; 1D) further comprises: a frequency counter for measuring the frequency (Fmes) of said radio frequency wave; and a radial beam position measuring device for measuring the orbit position (Rmes) of the charged particle beam, and a frequency counter for measuring the frequency (Fmes) of said radio frequency wave; and a radial beam position measuring device for measuring the orbit position (Rmes) of the charged particle beam, and the energy determination device is configured: to measure whether or not the energy of the charged particle beam orbiting within said circular accelerator differs from a predetermined energy value after the end of acceleration of the charged particle beam, and to determine that the energy of the charged particle beam orbiting after the end of the acceleration is normal, when the frequency (Fmes) of said radio frequency wave after the end of the acceleration is within a first allowable range (Ferr) and the orbit position (Rmes) of the orbiting charged particle beam after the end of the acceleration is within a second allowable range (Rerr). to measure whether or not the energy of the charged particle beam orbiting within said circular accelerator differs from a predetermined energy value after the end of acceleration of the charged particle beam, and to determine that the energy of the charged particle beam orbiting after the end of the acceleration is normal, when the frequency (Fmes) of said radio frequency wave after the end of the acceleration is within a first allowable range (Ferr) and the orbit position (Rmes) of the orbiting charged particle beam after the end of the acceleration is within a second allowable range (Rerr).