The background projection data of a background region CT image in the X-ray emission path is calculated. Using measurement projection data and background projection data, the measurement projection data of local regions is calculated. Local region CT images are calculated on the basis of local measurement projection data, and the projection data of a local region CT image in the X-ray emission path is calculated. On the basis of local calculation projection data and local measurement projection data, local CT images are iteratively corrected. When creating background region CT images or calculating background projection data, the cause of calculation accuracy deterioration is eliminated by the use of processing such as smoothing, and without deterioration of CT value accuracy in regions other than the target region, a high-quality CT image is obtained.
