PURPOSE: To attain the transmission of an auxiliary signal with a small circuit scale by giving a code rule violation to a 1B2B coding rule of a main signal by a position modulation pulse so as to superimpose the auxiliary signal onto the main signal.

CONSTITUTION: The input waveform of the auxiliary signal given to an input terminal 6 is subjected to pulse position modulation by a pulse position modulatio circuit 11 and a modulation pulse (b) whose time position with respect to a synchronizing pulse (a) is proportional to the sampling value of the input waveform is outputted. A retiming circuit 12 applies retiming the modulation pulse (b) by using a clock signal of a main signal and it is synchronized with the main signal and the result is fed to a CRV control input 15 of the coding circuit 1. The coding circuit 1, in the presence of the modulation pulse (b), applied code rule vioration (CRV) to the 1B2B code rule of the main signal and does not apply the CRV in other case. A synchronizing circuit 13 ANDs a CRV detection signal (e) and a 2-bit delay signal (f) retarding it by 2-bit to detect the synchronizing pulse. A demodulation circuit 14 demodulates the pulse position modulation by using the synchronizing signal (g). Thus, the auxiliary signal is to be sent with small circuit scale.