This invention has one object is to provide radiation tomography apparatus that allows production with low price through suppression in number of radiation detectors to be mounted. One of the detector rings in this invention is a first detector ring having a sufficient internal diameter to introduce shoulders of the subject M, and the other is a second detector ring having a smaller internal diameter than the first detector ring. In so doing, the radiation detectors forming the detector ring may be suppressed in number, which may provide radiation tomography apparatus of low price. Moreover, a smaller diameter of the detector ring may result in improved spatial resolution and detection sensitivity of radiation.