A gantry has a cylindrical shape and rotates around a subject on a top panel about the body axis. An X-ray irradiating part is arranged inside the gantry and emits an X-ray. An X-ray detector is arranged at a position facing the X-ray irradiating part and detects the X-ray transmitted through the subject. PET detectors are arranged in two separate regions facing the rotation center and detect .gamma.-rays emitted from the positron-emitting nuclides. A moving mechanism moves the top panel and the gantry relatively to each other. An X-ray CT image generator generates an X-ray CT image of the subject based on the result of detection by the X-ray detector. A PET image generator generates a PET image of the subject based on the .gamma.-rays detected by the PET detectors on the circumference in accordance with rotation of the gantry.