1. A method for creating an index for searching a structured document having a document data-structure stored, the method comprising the steps, to be performed by the computer, of: analyzing the structured document to extract the document data-structure; normalizing the extracted document data-structure to create a logical structure index composed of a plurality of elements having a hierarchical structure; extracting an appearance number of each element in the created logical structure index; and extracting elements for which an index is created, based on the logical structure index, by comparing the extracted appearance number and a first predetermined threshold.