A plurality of pieces of image data processed by a preprocessed circuit are sequentially received and at least one representative image is selected from among the plurality of pieces of image data by a representative image selection circuit. For example, one piece of image data out of every 250 pieces of image data is selected from among 1000 pieces of acquired image data to select four pieces of image data as representative images. An irradiated area extraction circuit extracts X-ray irradiated areas of the representative images. A reconstruction area determination circuit determines a CT reconstruction area on the basis of the X-ray irradiated areas. The reconstruction circuit reconstructs CT images from all or part of the acquired image data on the basis of the CT reconstruction area.