A document storing portion registers document data input from a document inputting unit to a document storing portion. A document search index creating portion analyzes the content of newly registered document data, creates a document search index that allows the document data to be searched for based on each tag, and stores the document search index to a document search index storing portion. A categorizing map creating portion refers to a categorizing map defined in a categorizing map storing portion, causes a document searching portion to search the document search index storing portion, creates leaves of the categorizing map, and creates a categorizing map for the newly registered document data. When the categorizing map is displayed, since only necessary leaves are displayed, the visibility and operability of the system are improved.