1. A case image search apparatus comprising: a cross-sectional image specifying device which specifies an arbitrary cross-sectional image obtained by cutting a three-dimensional image to be diagnosed with a cutting plane arbitrarily positioned and angled; a cross-sectional image producing device which produces a cross-sectional image from the three-dimensional image with reference to a diseased site in the specified arbitrary cross-sectional image, the cross-sectional image oriented in an axial direction set in advance and containing the diseased site; a feature value calculating device which calculates a first feature value of the diseased site contained in the produced cross-sectional image based on the cross-sectional image; a database which stores a plurality of case images and a second feature value of each of the case images, which is associated with a case image, the second feature value for diseased site in a cross-sectional image oriented in the axial direction set in advance; and a searching device which compares the first feature value with the second feature value and searches the database for a case image having a second feature value similar to the first feature value.