1. A structure detection apparatus for detecting a predetermined structure in image data, the apparatus comprising: a candidate point extraction means that extracts a plurality of candidate points belonging to the predetermined structure from the image data; a set form storage means that stores, in advance, a set form including a plurality of training labels, the set form representing a known form of the predetermined structure; a representative point selection means that selects, from the plurality of candidate points extracted by the candidate point extraction means, a plurality of representative points corresponding to the plurality of training labels respectively, the plurality of representative points included in a form model that is the same as or most similar to the set form stored in the set form storage means; and a structure detection means that detects the predetermined structure in the image data by using the form model including the plurality of representative points selected by the representative point selection means, and the plurality of candidate points, wherein the set form represents information about a connection relationship between the training labels, and wherein the representative point selection means selects the plurality of representative points by determining, for each of combinations of the training labels and the candidate points associated with each other, a path between two of the candidate points associated with two of the training labels that should be connected to each other by selectively tracing the plurality of candidate points including at least one candidate point that is not associated with the training label in the combinations in such a manner that a cost based on a predetermined index value becomes lowest, and by calculating a degree of correspondence between a connection relationship between the plurality of training labels and a connection relationship between candidate points associated with the training labels, respectively, by using a cost for the path, and by determining one of the combinations that has the highest degree of correspondence between the connection relationships.