The present invention is directed to improve the precision of a CT image of a local region (small FOV image) that is obtained by applying the extensive reconstruction technique to the iterative approximate reconstruction method. A first CT image relating to a first reconstruction range is reconstructed from the projection data of a subject, the projection data being detected by the X-ray detector of the X-ray CT apparatus, and the first CT image is corrected iteratively so that the first calculated projection data obtained from the first CT image according to projection calculation, becomes equal to the projection data of the subject. The first CT image thus iteratively corrected is used to extract local measured projection data in association with the second reconstruction range. A second CT image is reconstructed in the second reconstruction range and the second CT image is iteratively corrected so that the extracted local measured projection data becomes equal to the second calculated projection data that is obtained from the second CT image according to the projection calculation.
