1. A bone number determination apparatus comprising: an image storage means that stores a three-dimensional image composed of a plurality of tomograms obtained by imaging a human body; a body-of-sternum area extraction means that extracts a body-of-sternum area representing the body of sternum of the human body from the three-dimensional image; a rib area extraction means that extracts at least one rib area, each representing an individual rib, from the three-dimensional image; and a rib number determination means that determines the rib number of each of the at least one rib area based on a position of the body-of-sternum area extracted by the body-of-sternum area extraction means, the position corresponding to a lower border of the body of sternum, and the position of each of the at least one rib area.