An imaging device, or a PET device, opposed gamma camera type PET device, or open PET device in particular, that is combined with a radiation therapy device, in which detectors are rotated to reduce incidence of nuclear fragments on the detectors. For example, in an opposed gamma camera type PET device, beam irradiation and detector rotation can be synchronized to prevent the detectors from interfering with the treatment beam and reduce the incidence of nuclear fragments on the detectors. This makes it possible to reduce the incidence of nuclear fragments on the detectors without interfering with a treatment beam, thereby enabling measurement of annihilation radiations and three-dimensional imaging of the irradiation field immediately after irradiation or during irradiation.
