The apparatus for forming a document group structure data contains a base document specifying section that specifies plural documents to be positionally fixed on the tree structure as base documents based on an input from a user, an upper tree structure forming section that forms 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, a lower tree structure forming section that forms 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 a tree structure combining section that forms a tree structure data showing the hypertext structure by combining the upper tree structure data and the respective lower tree structure data.