An operating unit selects a piece of document data from among a plurality of pieces of document data from a document database. A relationship-information searching unit searches a relationship information database that stores relationship information in the form of parent-child relationship between the pieces of document data, for the relationship information pertaining to the selected piece of document data. A related-document-list creating unit creates from the retrieved relationship information, a list that indicates a hierarchical relationship between the selected piece of document data and other pieces of document data. A display unit displays the pieces of document data related to the selected piece of document data on a single screen based on the list.