PURPOSE: To display tags at all times even when plural pages are displayed one over another.

CONSTITUTION: The tag display controller which shifts the pages with the added tags from one another and displays them one over another has overlap order information and size information on the respective pages and appending position information and size information the tags corresponding to the pages in a display data table 4, and a tag control part 7 decides that the one of a lower page overlaps with an upper page or its tag according to the overlap order information and size information on the respective pages and the appending position information and size information on the respective tags; when the tag of the lower page is covered with the upper page or its tag, the tag control part 7 alters the appending position information and size of the tag of the lower page or reduces the size of the tag of the upper page, so that the tag of the lower page is also displayed at an output part 2 by the alterations of the appending position and size.