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 in that an artifact component caused by the image reconstructing method of the projection data is obtained by calculation; the obtained artifact component is subtracted from the projection data to create corrected projection data containing an inverted artifact component, and image reconstruction is executed on the corrected projection data to obtain a reconstructed image from which the artifact is reduced.