An image-processing method for compressing image data, includes: (a) obtaining a block composed of part of or all of the image data; (b) determining whether the obtained block is color or monochrome; and (c) performing, in the case where the obtained block is determined to be color, compression processing in parallel for a predetermined number of elements of a predetermined color system, and performing, in the case where the obtained block is determined to be monochrome, the compression processing in parallel for individual areas obtained by dividing a division block containing the obtained block into a predetermined number of pieces for a single component of the predetermined color system.