PURPOSE: To connect directly a transmitting/receiving device to a transmission line, by providing an nBmB conversion part between the synchronization part and parallel/series conversion part of a transmitting device and also providing a nBmB reverse conversion part between the series/parallel conversion part and destaff part.

CONSTITUTION: The synchronization part 11' receives (n) low group signals from input terminals 1'-1∼1'-n and sends synchronizing signals in an (n) series to the nBmB conversion part 12'. The nBmB conversion part 12' converts these (n) series synchronizing signals into (m) series signals by parallel processing. Consequently, a balance code necessary as a transmission line code is obtained. The parallel/series conversion part 13' multiplies the (m) series signals and sends one series signals from an output terminal 14' to a direct transmission line.