In order to provide an aperture position adjusting mechanism in an X-ray CT system with low cost and high accuracy to enable stable reconstruction of X-ray tomographic images, a first shaft (23) is provided with a bore (23a) at a position offset from a center axis, and is rotatably supported by a base plate (20) orthogonally to a pair of rails (21a, 21b). A second shaft (25) is received and is rotatably supported within the bore (23a) through the first shaft (23). The aperture (6) is moved along the rails (21a, 21b) as the second shaft (25) is eccentrically rotated by a motor (7) and, following the eccentric rotation and in a direction opposite to that of the rotation, the first shaft (23) is eccentrically rotated around the center axis of the bore (23a). |