PURPOSE: To provide a compression method for pattern data in which all data are expressed in a byte and data with a small consecutive number are processed at a high speed with high efficiency.

CONSTITUTION: Consecutive white dot image whose consecutive number is 7 or less or a consecutive black dot image is pattern data, a non-compression index data are written in a 7th bit being a most significant bit of bits of 1 byte and white bit data or black bit data are written into each bit below the 6th bit according to the arrangement of pattern data, and in the case of consecutive white dot image or black dot image whose consecutive number is 8 or over, compression index data are written in the 7th bit, white dot image index data or black dot image index data are written in 6 bits and a run length conversing dot image data into binary data is written in the 5 bits or below and the pattern data are compressed to be expressed in one run length and bit data.