1. A watermark information embedding apparatus, comprising: a document image generating section for generating a document image; a PN code generating section for generating a plurality of PN code sequences; a watermark image generating section for diffusing units of watermark information using the PN code sequences, generating representational watermark information, and generating a watermark image in which the diffused units of watermark information are redundantly denoted by dot patterns that are repeated at a plurality of locations; a synthesizing section for overlapping the document image and the watermark image so as to generate a watermarked document image; and a printer that prints the watermarked document image on a recording medium wherein the units of watermark information are represented by bits, wherein the watermark image generating section diffuses each bit of watermark information by representing the respective bit by one of the PN code sequences if the respective bit has a first value and by representing the respective bit by a modified version of the one of the PN code sequences if the respective bit has a second value, and wherein the watermark generating section uses a first one of the PN code sequences and a modified version of the first one of the PN code sequences for all of a first row or column to diffuse watermark information and uses a second one of the PN code sequences and a modified version of the second one of the PN code sequences for all of a second row or column to diffuse watermark information.