1. A method for compressing image data that are processed by a pseudo gradation process using a dither pattern, the method comprising the steps of: dividing the image data into predetermined areas; determining whether or not a dot arrangement pattern of the image data in each area matches the dither pattern that was used in the pseudo gradation process; determining a representative value concerning density of the image data in the area if the dot arrangement pattern matches the dither pattern; and making compressed data by using the determined representative value.