15. A feature extraction method comprising: searching a document tree having a plurality of elements according to a rule given in advance, and sequentially detecting said an element of said plurality of elements as a search element; calculating an inter-element distance between an extraction target element specified in advance among a plurality of elements of said document tree and said search element; generating exclusivity information which indicates whether said search element is an exclusive element to said extraction target element, with reference to an exclusive element name specified in advance; calculating a weight for a phrase included in the element corresponding to said element based on the inter-element distance and said exclusivity information, and calculating an element feature vector, having a plurality of dimensions and each dimension uniquely corresponding to a predetermined phrase, based on the calculated weight corresponding to each search element; and calculating a partial document feature vector of a partial document related to said extraction target element based on said element feature vector.
16. The feature extraction method according to claim 15 , wherein said generating of exclusivity information includes, referring to an element name of said extraction target element as an exclusive element name.