1. An X-ray therapy system comprising: a ring-shaped gantry in which a patient is arranged; a positron emission tomography (PET) 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 pair of photon detection units arranged to interpose the patient and to be opposed to each other, 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 connected to each of the pair of photon detection units and the gantry and configured to move the pair of photon detection units 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, and wherein the photon detection unit-moving device is configured to arrange the pair of photon detection units at positions separated from the patient by a predetermined distance when detecting photons in the PET and is configured to move the pair of photon detection units to refraction positions separated from the patient by a distance which is greater than the predetermined distance before irradiation of the X-rays starts when the X-rays are radiated from the irradiation unit to the patient.