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; and a calculation unit configured to calculate frequency-of-occurrence information indicating locations of each of the lexical items in the acquired structured document.