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, wherein the X-ray CT image of the inside of the subject is captured after the moving mechanism displaces the top panel and the PET image of the inside of the subject is captured after the moving mechanism displaces the top panel, the capturing timing of the X-ray CT image being different from the capturing timing of the PET image, and wherein the moving mechanism rotationally moves the gantry so that a mode of rotationally moving the gantry is different between capturing of the X-ray CT image and capturing of the PET image.