Even when character information is contained in a high-resolution image, it is possible to reduce the line memory capacity and effectively encoding/compressing the image. For every block update for an image divided into identical M (M is an integer not smaller than 1) blocks in the horizontal direction, for each of the horizontal lines in the blocks, a line memory (41) and an exclusive OR circuit (42) successively detect a difference between color codes at corresponding pixel positions in a horizontal line and the adjacent preceding horizontal line. Moreover, a run end detection circuit (43) detects a run from the difference corresponding to the horizontal line from the circuit (42). When a code creation/output circuit (44) encodes the run length, the code word for the first run includes information on the difference in the run while the code word for runs other than the first run includes, instead of the difference information in the run, transition type information indicating the type of transition to the difference in the run of the difference in the preceding run, thereby performing run length encoding.