1. An image data compressing method comprising: extracting image elements included in a digital image from a sheet of paper, said extracted image elements including a free microdot image element, a print dot image element, and a line picture image element; compressing said extracted image elements with compressing methods respectively corresponding to kinds of said image elements to generate compressed image element data, wherein the print dot image element is comprised of dots arranged in accordance with a print rule, each of the dots having an area corresponding to a graduation, and wherein the free microdot image element is comprised of free microdots which are not arranged in accordance with the print rule, each of the free microdots having an area equal to or less than a predetermined area.