A combined radiation therapy/PET apparatus includes: an open PET device having multi-ring detector rings that are opposed to each other in the direction of the body axis so as to leave a gap therebetween; and a radiation therapy apparatus for performing radiation therapy through the gap. When imaging the condition of an affected area and a treatment beam for monitoring in radiation therapy of irradiating the affected area with X-rays, gamma rays, or particle beams, the apparatus covers a region of interest in the irradiation field of the radiation therapy with the field-of-view of the open PET device, thereby making possible the positioning of the irradiation field and treatment monitoring using PET images.