According to one embodiment a radiation therapy apparatus includes a radiation irradiation device (600), a detector included in a PET scanner (200), a control unit (540), and a PET image reconstruction unit (525). The radiation irradiation device (600) emits a therapeutic radiation. The detector counts light derived from gamma rays, and is provided with a gap portion through which the therapeutic radiation emitted by the radiation irradiation device (600) passes on a plane of rotation about the body axis of a subject. The control unit (540) controls the radiation irradiation device (600) 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 (525) reconstructs a PET image based on position information at the time of counting of the detector that nearly coincidentally counts pair annihilation gamma rays emitted in association with release of energy of a heavy particle beam in a state where the control unit (540) performs rotation control.
