An image-constituting element group corresponding to bone areas is selected from among a plurality of image-constituting elements that make up a three-dimensional image model, and CT values originally associated only with the selected image-constituting element group are replaced by values resulting from multiplying the original values by ¼. As a result, a distribution curve 34A denoting a frequency distribution of bone is shifted to a region of CT values that are lower than those of a distribution curve 33 denoting a frequency distribution of blood vessels. An observation image is then constructed by maximum intensity projection on the basis of three-dimensional image data after such signal value replacement.