1. A watermark information embedment device comprising: a document image formation portion forming a document image based on the document data on the page by page basis; a watermark image formation portion forming a watermark image; and a watermarked document image composition portion forming a watermarked document image by putting said document image on said watermark image; wherein said watermark image is made up of plural sorts of dot patterns well-regularly arranged according to a certain rule, and at least one sort of said dot patterns is provided with secret information indicative of a specific confidential matter.