PURPOSE: To compress a dot pattern and to attain the reducing of information quantity by storing the dot pattern of an objective block in a real data area only when the objective block and an adjacent block disagree.

CONSTITUTION: The comprarison result area 31 of a compressed data storage part 3 is an area storing bit groups indicating a coincidence or a non-coincidence at every block divided at every prescribed dots in the row direction of the dot pattern before the compression and the real data area 32 of the part 3 is an area storing dot patterns of non-coincident blocks when blocks into which dot patterns are divided do not coincide with corresponding blocks of adjacent rows. Then, a comparison processing control part 4 decides whether a block divided at every prescribed dots in the row direction of the dot pattern coincides or nonocoincides with an adjacent block and then stores the decided results in the comprarison area 31 and further at the time of the non-coincidence stores the pattern of the block in the real data area 32.