PURPOSE: To speed up a transmission time by fetching a data desired to be transmitted and a data transmitted to a transmission line just before to two registers, detecting repetitive information by a coincidence circuit, replacing a repetitive data with a simple data and transmitting it as a serial data.

CONSTITUTION: A data 101 is transferred from a communication control section (CCU)1 to a register 2, from the register 2 to a register 3 in the unit of bytes via a parallel path respectively. The coincidence circuit 4 compares outputs the registers 2, 3 and transmits a decision signal whether or not the data is coincident in the unit of bytes to a bit shift counter circuit 8. A repetitive recognition code "00" in 2-bit is transmitted to the circuit 5 by a repetitive recognition code addition circuit 6 depending on the decision signal representing [dissidence] and then transmitted to a synchronism bit addition circuit 7 as a serial data 102 in 10-bit. On the other hand, the circuit 6 transmits the repetitive recognition code "01" in 2-bit to the circuit 5 in response to the decision signal representing [coincidence].