1. An XML document classifying method, for extracting from among a plurality of XML documents, stored in a storage system, only those XML documents that include an element (hereinafter referred to as a initial element) having a first arbitrary character string (hereinafter referred to as a first initial character string) as a name, comprising the steps of: (1) classifying, based on the identify of the character string (hereinafter referred to as an element value), XML documents that have only a child of the initial element that is a character string; (2) generating a folder (hereinafter referred to as a initial element folder) having the same name as the initial element, and arranging, for the individual element values, lower folders (hereinafter referred to as element value folders) that employ the element values as names immediately under the initial element folder, and generating a folder hierarchical expression wherein XML documents, the element values of which are the same as the names of the element value folders, are arranged immediately under the individual element value folders; and (3) when one of the element value folders is selected and a second initial character string is designated, extracting from the XML documents in the element value folders documents that include an element (hereinafter referred to as a second element) having the second initial character string as a name, regarding the second initial element as a initial element to perform steps (1) and (2), and generating a hierarchical expression wherein folders that are generated at step (2) and have the same name as the second initial element are arranged immediately under the element value folders, whereby a folder hierarchial expression generated by recursive repetition of steps (1) to (3) is provided.