With the objective of evaluating whether a dose is excessive for a subject, such a dose that image noise reaches less than or equal to a predetermined value is estimated upon an axial scan on the basis of projection data acquired by a scout scan. The dose is set as a maximum dose. An area other than air, of an image obtained by the axial scan is partitioned into a plurality of small areas i. Image noises N(i) at the respective small areas i are calculated. A warning image G 1 in which all of pixel values of pixels in small areas in which the image noises N(i) are less than or equal to a predetermined value, are substituted with pixel values expressed in black level, is created and displayed.