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 an imaging-area setting unit that sets, on the subject, a first imaging area including a first imaging region that moves cyclically or that moves non-cyclically, and a second imaging area including a second imaging region having less movement than the first imaging region, wherein the imaging-area setting unit further sets, within the first imagining area, a third imaging area requiring an image having a predetermined temporal resolution and a fourth imaging area requiring an image having a better temporal resolution than the predetermined temporal resolution, and the image reconstruction unit reconstructs a first 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, increases a frequency of synchronization during reconstructing the first image compared to when the top plate moving unit moves the top plate to a position at which the fourth imaging area is irradiated with the X-rays, when the top plate moving unit moves the top plate to a position at which the third imaging area is irradiated with the X-rays, and reconstructs a second 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.