A method for compressing image data on which a pseudo gradation process has been performed by using a dither pattern is provided. The method includes the steps of dividing the image data into predetermined blocks, calculating a typical value if an arrangement pattern of image data in each block can be reproduced by using a dither pattern that was used for the pseudo gradation process and the typical value, checking whether there is an arrangement pattern that matches the arrangement pattern of the image data in the block before then if it cannot be reproduced, calculating position data indicating a matching position if there is the arrangement pattern before then, obtaining the arrangement pattern of the image data in the block as block data if there is not the arrangement pattern before then, and using the calculated or obtained typical value, position data or block data for generating compressed data.