In beam monitoring for detecting annihilation radiations produced by radiation irradiation in radiation therapy for cancer which is performed by irradiating the affected area by X-rays, gamma rays, or particle beams, a detector-shift type combined radiation therapy/PET apparatus is provided with an open PET device that includes a plurality of shiftable multi-ring detector rings; and a radiation irradiation device that is capable of irradiation with a radiation beam through between the detector rings. The apparatus changes the positions of the detector rings, performs irradiation with the radiation beam through between the detector rings, and then performs radiation measurement.