An organism is irradiated with therapeutic radiation from a radiation irradiation device. A pair of two-dimensional radiation detectors are arranged so as to face one another with the irradiated therapeutic radiation passing therebetween, and detect the two-dimensional positions irradiated by a pair of annihilation γ rays produced when a positron emitted from a positron-emitting radionuclide is annihilated. On the basis of a pair of positions detected by the pair of two-dimensional radiation detectors, a radionuclide position detecting unit detects the position of the positron-emitting radionuclide, and the radiation irradiation device irradiates the position of the positron-emitting radionuclide with therapeutic radiation.
