PURPOSE: To realize a large throughput by providing a data compressor which compresses print data between an input and an output terminal without changing the code system of the print data and sends the compressed data out to the output terminal.

CONSTITUTION: The control part 3 of the adapter for the printer consists of the microcomputer composed of an MPU 4, a ROM 5, a RAM 6, etc., and a compression LSI 8, a display part 9, a switch part 11, and parallel interfaces 12W13 are connected to its internal bus 7. Thus compression LSI 8 is provided so as to reduce the load on the MPU 4 and executes the algorithm of data compression. Further, when the LSI 8 processes the data, the data to be compressed are inputted to a register in order and then compressed and outputted in order to a register. This adapter is provided between a personal computer or word processor and the printer to shorten the printing-out time.