The present invention attempts to reduce an amount of a contrast medium or a patient dose and improve diagnostic efficiency. The conditions for a main scan are designated so that a first scan of scanning a subject with a tube current of a first tube current value fed to an X-ray tube will be performed in order to examine the blood flow in a subject's blood vessel into which a contrast medium is injected. Moreover, the conditions for a main scan are designated so that a second scan of scanning the subject with a tube current of a second tube current value fed to the X-ray tube will be performed in order to image the subject's blood vessel into which the contrast medium is injected. Images of the subject are constructed based on projection data items produced by scanning the subject under the sets of conditions for a main scan.