PURPOSE: To increase the transmission enable range by deleting/ adding the information quantity.

CONSTITUTION: A transmission buffer 10 fetches information from a register 6 based on a timing signal from a selective circuit 7. When the signal is a timing signal from a timing generating circuit 3 at this time, transmission control information D and voice data information sets B1, B2 are all fetched by the register R 6. They are all sent to a driver 12 and then to a transmission line. A reception buffer 11 fetches information received by a receiver 13 based on a signal from a selective circuit 8 and is written in the register R 9. When the selective signal is a signal from the circuit 3, the information sets D, B1, B2 are all fetched by the buffer 11 and written in the R 9. Thus, the delay quantity to be permitted from the transmission line is increased to increase the transmission enable distance.