Slice image groups of a first photographing opportunity and a second photographing opportunity are acquired. Three-dimensional tumor areas (lesion areas) are extracted from the slice image groups, and a slice image with maximum tumor area is determined as a reference slice image of each slice image group. Based on the reference slice images thus determined, slice images positioned at the same distance away from the reference slice images in the slice direction are respectively acquired from the slice image groups of a first photographing opportunity and a second photographing opportunity, and the acquired slice images are displayed on a monitor device so that images can be compared and interpreted. Thus, an image interpreter can accurately compare and interpret images of lesion areas included in slice image groups with different photographing opportunities.