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 a dot arrangement pattern that is obtained by a binarization process that uses imaginary uniform image data and the dither pattern; determining a representative value concerning density of the image data in the area if the dot arrangement pattern of the area matches the dot arrangement pattern for the imaginary uniform image data; and making compressed data by using the determined representative value; wherein the representative value is a value “a” that satisfies the condition J≧a≧K, wherein a dot for a given pixel is formed if a density value of the given pixel is smaller than a density value of a pixel in a corresponding part of the dither pattern, “J” is a minimum value of upper limit density values of pixels of the dither pattern corresponding to the dots that were formed in the area and “K” is a maximum value of lower limit density values of pixels of the dither pattern corresponding to dots that were not formed in the area; wherein the steps are performed by a computer.