In the case of editing document data that has a plurality of fields, by allowing the user to identify a field that is closely related to the selected field, operability of the editing operation of the document data is improved. A selection field detecting unit detects one of the fields according to the operation of the user. An automatic level allocating process executing unit decides the relation degrees between the detected selection field and the other fields. A field enlarging/contracting unit and a display screen control unit switch display control regarding those other fields based on the relation degrees.