X-ray CT apparatus, method of aligning phantom, and phantom retaining tool

The present invention comprises a top plate for placing a cylindrical phantom , an X-ray tube that generates X-rays, an X-ray detector that detects X-rays transmitted through the phantom placed on a top plate , a supporting body drive part that rotates the X-ray tube and X-ray detector , a tomographic image data generating part that generates tomographic image data on the phantom based on a results of detecting X-rays by the X-ray detector , a calculation processing part that calculates displacement of the cylinder axis J of the phantom relative to the center of rotation (center of scan) by the supporting body drive part , based on the tomographic image data, and the angle of gradient of the cylinder axis J relative to the normal direction (direction of slice) of the rotation plane of said rotation, and a monitor that displays the calculated results.