1. A structured document processing apparatus comprising: an acquisition unit configured to acquire a structured document; a storage unit configured to store a structure model tree which indicates a typical structure of the acquired structured document; a parsing unit configured to parse the acquired structured document; an updating unit configured to update the structure model tree to match a structure of the parsed structured document therewith; a division unit configured to divide the acquired structured document into a plurality of lexical items; a calculation unit configured to calculate frequency-of-occurrence information indicating locations of each of the lexical items in the acquired structured document; a broadening unit configured to broaden a range until a lexical item having not less than a frequency of occurrence is present within the range; and an assignment unit configured to assign a lexical identifier of a lexical item which has a highest frequency of occurrence within the broadened range as a relevant lexical identifier of the lexical item.