An X-ray therapy system comprising: a ring-shaped gantry in which a patient is arranged; a positron emission tomography detection device provided in the gantry and configured to perform PET on the patient; an irradiation unit provided in the gantry and configured to radiate X-rays to the patient; and a rotation-driving unit for causing the gantry to perform a rotational movement around a predetermined rotation center, wherein the PET detection device includes: a photon detection unit to detect photons emitted from the patient based on nuclear decay of a tracer nuclide administered to the patient; and a photon detection unit-moving device configured to move the detection unit with respect to the gantry, wherein the rotation-driving unit causes the irradiation unit and the PET detection device to perform the rotational movement around the predetermined rotation center by causing the gantry to perform the rotational movement around the predetermined rotation center, characterized in that the gantry is provided with openings to form a pair of concave portions on an inner circumferential surface of the gantry, wherein the concave portions include bottom surfaces which are inside the concave portions and opening portions which are on the inner circumferential surface of the gantry, wherein the opening portions of the concave portions are provided with cover members which are configured to block the X-rays or the scattered rays due to the X-rays from the irradiation unit, wherein the photon detection unit includes a pair of photon detection units disposed at opposite sides to each other with the patient therebetween, wherein the photon detection unit-moving device includes photon detection unit-moving devices connected to each of the pair of photon detection units, wherein a first end of the photon detection unit-moving device is fixed to the photon detection unit, and a second end of the photon detection unit-moving device is fixed to the bottom surface of the concave portion, wherein when detecting photons in the PET, the photon detection unit-moving devices are configured to arrange the photon detection units at positions separated from the patient by a predetermined distance, and wherein when the X-rays are radiated from the irradiation unit to the patient, the photon detection unit-moving devices are configured to move the photon detection units away from the patient and to house them in retraction positions inside the concave portions before irradiation of the X-rays starts. a ring-shaped gantry in which a patient is arranged; a positron emission tomography detection device provided in the gantry and configured to perform PET on the patient; an irradiation unit provided in the gantry and configured to radiate X-rays to the patient; and a rotation-driving unit for causing the gantry to perform a rotational movement around a predetermined rotation center, wherein the PET detection device includes: a photon detection unit to detect photons emitted from the patient based on nuclear decay of a tracer nuclide administered to the patient; and a photon detection unit-moving device configured to move the detection unit with respect to the gantry, a photon detection unit to detect photons emitted from the patient based on nuclear decay of a tracer nuclide administered to the patient; and a photon detection unit-moving device configured to move the detection unit with respect to the gantry, wherein the rotation-driving unit causes the irradiation unit and the PET detection device to perform the rotational movement around the predetermined rotation center by causing the gantry to perform the rotational movement around the predetermined rotation center, characterized in that the gantry is provided with openings to form a pair of concave portions on an inner circumferential surface of the gantry, wherein the concave portions include bottom surfaces which are inside the concave portions and opening portions which are on the inner circumferential surface of the gantry, wherein the opening portions of the concave portions are provided with cover members which are configured to block the X-rays or the scattered rays due to the X-rays from the irradiation unit, wherein the photon detection unit includes a pair of photon detection units disposed at opposite sides to each other with the patient therebetween, wherein the photon detection unit-moving device includes photon detection unit-moving devices connected to each of the pair of photon detection units, wherein a first end of the photon detection unit-moving device is fixed to the photon detection unit, and a second end of the photon detection unit-moving device is fixed to the bottom surface of the concave portion, wherein when detecting photons in the PET, the photon detection unit-moving devices are configured to arrange the photon detection units at positions separated from the patient by a predetermined distance, and wherein when the X-rays are radiated from the irradiation unit to the patient, the photon detection unit-moving devices are configured to move the photon detection units away from the patient and to house them in retraction positions inside the concave portions before irradiation of the X-rays starts.