PURPOSE: To prevent a compression effect from being lowered even when a lot of short white or black picture element lines are generated by mixing non- compressed data and compressed data on one scanning line of image data.

CONSTITUTION: An input control part 11 inputs the image data and judges the continuation of white or black picture elements. When the picture elements are continued over 5 bits, they are transmitted to a compression processing part 12 and when the picture elements are continued less than 4 bits, they are transmitted to a non-compression processing part 13. The compression processing part 12 executes the compression processing of data by a one-dimensional encoding system, and the non-compression processing part 13 outputs the input data to an output control part 14 as they are. According to the order of the input data, the output control part 14 builds up the compressed data and the non- compressed data and outputs them after adding an identification code. Thus, even when a lot of short white or black picture element lines are generated, the compression effect is not lowered.