1. An X-ray CT image forming method that obtains a reconstructed image of an examination site of an examinee by executing image reconstruction on projection data obtained by CT scan, characterized by comprising: (1) a step of executing image reconstruction on the projection data to obtain an initial reconstructed image; (2) a step of re-projecting the initial reconstructed image to obtain re-projection data; (3) a step of obtaining differential data between the projection data and the re-projection data; (4) a step of subtracting the differential data from the projection data to obtain corrected projection data containing an inverted artifact component; and (5) a step of executing image reconstruction on the corrected projection data to obtain a corrected reconstructed image.