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: a processor configured to: specify a plurality of documents to be positionally fixed on the tree structure as base documents based on an input from a user; form 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; and form a 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, determining whether or not a linked document detected upon following the links of the hypertext structure starting from one of the base documents is incorporated into the lower tree structure with the base document as a root node, and determining to incorporate the linked document into the lower tree structure of the base document as a root node in a case where, among directories storing the plurality of base documents, a directory storing the base document as a starting point is the nearest ancestor with respect to a stored position of the linked document in the hierarchical directory structure; form a tree structure data showing the hypertext structure by combining the upper tree structure data and the respective lower tree structure data, and at least one of an output device, display, or storage device for outputting, displaying or storing the tree structure data.