A data compressing method which can efficiently compress image data containing color data is provided. A compressed data expanding method which can expand the compressed data at high speed is provided. Image data containing color data are compressed into image data including: address fields code A, code B, code C which hold addresses of color RAMs storing color information corresponding to the color data; classification fields which, when the color RAMs are classified into prescribed categories A, B, C, hold classification information of the categories including the color data; and run length fields RL which hold numbers of successive repetitions of color data. To expand the compressed image data, the classification fields are decoded to determine the categories in which the color data are included. Based on the determined categories, the first processing for decoding the address fields to generate addresses, and the second processing for supplying the generated addresses by the numbers of successively repeated color data in the run length fields to access the color RAMs are performed.