1. An apparatus for forming a document group structure data that forms a tree structure data showing a tree structure corresponding to a hypertext structure constituted by a document group contained in a hierarchical directory structure, based on the hypertext structure, the apparatus for forming a document group structure data comprising: base document specifying means for specifying a plurality of documents to be positionally fixed on the tree structure as base documents based on an input from a user; upper tree structure forming means for forming an upper tree structure data showing a hierarchical relation among the base documents based on positional information of the base documents in the directory structure; lower tree structure forming means for forming lower tree structure data showing tree structures of document groups having the base documents as root nodes, respectively, by following links of the hypertext structure starting from the respective base documents; and tree structure combining means for forming a tree structure data showing the hypertext structure by combining the upper tree structure data and the respective lower tree structure data.