PROBLEM TO BE SOLVED: To simply select an optimum combination of CT images to be subjected to difference processing.SOLUTION: A storage part 13 extracts an X-ray high absorption area about X-ray high absorption substance from data on a non-contrast volume image of at least one time phase included in a non-contrast volume image group and data on a contrast volume image of a plurality of time phases included in a contrast volume image group. A parameter calculation part 17 calculates a feature amount that fluctuates in accordance with the movement of the X-ray high absorption area about each non-contrast volume image of at least one time phase and each contrast volume image of a plurality of time phases. An image selection part 19 selects a first non-contrast volume image and the contrast volume image in which image features of the X-ray high absorption areas are similar from among non-contrast volume images of at least one time phase and the contrast volume image of a plurality of time phases on the basis of the calculated feature amount.
