According to one embodiment, a radiation therapy apparatus includes a radiation irradiation device, a detector included in a PET scanner, a control unit, and a PET image reconstruction unit. The radiation irradiation device emits a therapeutic radiation. The detector counts light derived from gamma rays, and is provided with a gap portion through which the therapeutic radiation passes on a plane of rotation about the body axis of a subject. The control unit controls the radiation irradiation device and the detector so as to rotate in synchronization with each other in a state capable of emitting the therapeutic radiation to the gap portion. The PET image reconstruction unit reconstructs a PET image based on position information at the time of counting of the detector that nearly coincidentally counts pair annihilation gamma rays in a state where the control unit performs rotation control.