1. An electronic watermark embedding apparatus that embeds watermark information as digital information into a document image, the apparatus comprising: a background determining unit that determines whether pixels in the document image are pixels composing a background image, wherein the background determining unit includes: a region extracting unit that examines adjacent pixels, and groups adjacent pixels that have a feature in common into one of at least one extracted region, and a region determining unit that determines whether each at least one extracted region is a region of the background image and has pixels that are to be included in the background image; a background eliminating unit that eliminates the pixels determined to be included in the background image by the background determining unit from the document image; and a watermark embedding unit that embeds a watermark into a background eliminated image from which the background is eliminated by the background eliminating unit.