<p>An X-ray CT apparatus in which an X-ray source (6) and an X-ray detector (7) mounted on a guide arm (3) are movable in directions of approaching to and separating from a sample (S). Against its movement, the center of gravity of the entire including constituent elements mounted on the guide arm (3) is kept on a rotating axis by a weight balance adjusting mechanism. The weight balance adjusting mechanism includes a movable weight (12) movably mounted on the guide arm (3). The movable weight (12) is moved and controlled so that the difference between rotational moment about the rotating axis (O) can be offset to approximately zero. A moving route for the movable weight (12) is set an optional straight line which is parallel with the moving direction of the X-ray source (6) or the X-ray detector (7) and doesn't pass through the rotating axis (O).</p>
