An arbitrary cross-sectional image (MPR image) is used as an image for searching a case DB which has registered case images for a case image similar to an image to be diagnosed. A user terminal produces an axial image with reference to a diseased site in the MPR image, the axial image containing the diseased site, calculates a feature value (first feature value) of the diseased site contained in the axial image, and sends the feature value to the case DB. A case image server compares the received first feature value with a feature value (second feature value) for each case registered in a feature value DB, searches the case DB for a case image having a second feature value similar to the first feature value based on the comparison result, and sends the searched case image to the user terminal.