A radiation tomograph whereby the acquisition of transmission data and the measurement of annihilation radiation pairs are simultaneously performed by a single detector ring so as to realize both reduction of the manufacturing cost of a PET device and reduction of a burden to a subject. The transmission data, which shows the distribution of annihilation radiation absorption characteristics within a subject, is computed from data relating to annihilation radiation pairs in the vicinity of a surface of the subject. The transmission data can be acquired by detecting a radioactive drug derived from the subject, which makes imaging exclusively for transmission data unnecessary.