1. A computer-implemented feature extraction device comprising: a search unit which searches a document tree having a plurality of elements according to a rule given in advance, and sequentially detects an element of said plurality of elements as a search element; a distance calculation unit which calculates 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; an exclusive element confirmation unit which generates 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; an element feature vector calculation unit which calculates a weight for a phrase included in the element corresponding to said element based on the inter-element distance and said exclusivity information, and calculates 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 a partial document feature vector calculation unit which calculates a partial document feature vector of a partial document related to said extraction target element based on said element feature vector.