1. A PET device comprising: a top panel on which a subject to which an agent labeled with positron-emitting nuclides have been administered is placed; a cylindrical gantry configured to rotate around the subject about a body axis of the subject; an X-ray irradiating part arranged inside the gantry and configured to irradiate an X-ray; an X-ray detector arranged at a position facing the X-ray irradiating part inside the gantry and configured to detect the X-ray having been transmitted through the subject; two PET detectors arranged in two regions facing a rotation center inside the gantry and configured to detect γ-rays emitted from the positron-emitting nuclides; a moving mechanism configured to move the top panel and the gantry relatively to each other; an X-ray CT image generator configured to generate an X-ray CT image of an inside of the subject based on a result of detection by the X-ray detector; and a PET image generator configured to generate a PET image of the inside of the subject based on the γ-rays detected by the PET detectors on a circumference in accordance with rotation of the gantry.