12. A document management method comprising: storing, in a document storage unit, pieces of document data; storing, in a relationship-information storage unit, relationship information indicating a parent-child relationship between the pieces of document data stored in the document storage unit; selecting a piece of document data from among the pieces of document data stored in the document storage unit as a selected piece of document data; searching for related-document information that is relationship information pertaining to the selected piece of document data; stopping the searching when an upper limit for the relationship information is reached; and creating, from the related-document information retrieved at the searching, a list that indicates a hierarchical relationship between the selected piece of document data and other pieces of document data stored in the document storage unit.
13. The document management method according to claim 12 , further comprising displaying the pieces of document data that are related to the selected piece of document data on a single screen based on the list.