A particle beam therapy device that irradiates a patient with a cation beam to perform treatment, the device including a passage selection unit that selectively passes through the cation beam among a mixed beam in which the cation beam and other species of a beam having a nuclide different from that of the cation beam are mixed after passing through a deflection magnetic field, after causing the mixed beam to pass through the deflection magnetic field, in a case where the other species of the beam is generated from the cation beam.