A detector ring has a first unit and a second unit that are rotatably movable and an auxiliary unit movable in a central axis direction. The auxiliary unit is moved in the central axis direction relative to the units. The detector ring forms into a C shape to form a clearance. Subsequently, the first unit and the second unit are rotatably moved to move the clearance of the detector ring to a position distant from a support member. A patient can be introduced from the clearance. The detector ring is returned to an annular shape in the reverse sequence of the above one with the patient introduced. A annihilation radiation pairs can be detected from all directions to take a tomographic image, and a functional image with higher image quality than that of a conventional configuration can be obtained.