A method for compressing/restoring image data free from degradation of image quality caused by compression/decompression, a method for compressing image data at high compression ratio, and its restoring method are disclosed. In the image data compressing method, image elements contained in a digital image read from a sheet of paper are extracted (S02), each of the extracted image elements (51-53) is subjected to data compression by a compression method corresponding to the type of the image element (S03-S05), and the compressed image element data (54-56) are stored.