PURPOSE: To speed up decoding through search in changing point at a reception side, by sectioning one line into blocks in the unit of prescribed bit length and adding a flag bit indicating the changing point in a block.

CONSTITUTION: An expansion start signal S generated with incoming of a compressed data block drives an address counter 31. One block's share of data stored in a line memory 41 is stored in the register 51 of the next stage. The data of the next coded line is reproduced by regarding the reference block data after coding as a base. Search of the changing point with respect to the reference data is performed when the flag bit in the flag bit register 53 is set and the changing point bit in the reference line is detected with an FF54 and an EOR 55. Based on the changing point bit of the reference line, the coded line is encoded and sequentially stored in the coded line memory 42 via a bus 61.