1. An X-ray CT apparatus comprising: a top plate moving unit that stepwisely moves a top plate on which a subject is placed in a body axis direction; a rotation mechanism unit that rotates an X-ray irradiating unit on a circular orbit approximately centering on the subject for every time the top plate moving unit moves the top plate; a data collecting unit that collects projection data obtained by detecting X-rays passing through the subject; an image reconstructing unit that reconstructs an image from the projection data collected by the data collecting unit; and a first imaging-area setting unit that sets, on the subject, a first imaging area including a first imaging region that moves cyclically or non-cyclically, and a second imaging area including a second imaging region having less movement than the first imaging region, wherein the image reconstruction unit reconstructs the image synchronously with movements of the first imaging region when the top plate moving unit moves the top plate to a position at which the first imaging area is irradiated with the X-rays, and reconstructs the image without synchronizing with movements of the second imaging region when the top plate moving unit moves the top plate to a position at which the second imaging area is irradiated with the X-rays.