PURPOSE: To minimize the number of the wiring by installing the modulation/demodulation unit, which gives the modulation and multiplication to the fixed plural units of signals and also gives the isolation and demodulation to the received PAM multiple signal, to the transmitter/receiver units at two spots separated from each other.

CONSTITUTION: The analog signals which are supplied from transmission signal input terminals IN1∼IN8 and via MPX/DMPX1 are sampled and then multiplied after PAM modulation to be transmitted to transmitter/receiver unit B of the other side via signal cable L1 and in the form of the PAM multiple signal. Then the multiple signal introduced into MPX/DMPX2 which is controlled in synchronization with DMPX1 is divided into the PAM signal with every channel and then extracted to each of corresponding output signal lines l'1∼l'8. While the PAM signal is held at the charge holding circuit comprising capacitor element C'1 and voltage follower V.FOL1' and filtered through the low pass filter after converted into the stair step waveform and then demodulated into the analog signal to be delivered through output terminal OUT1'. The same demodulation action mode is given also to other output signal lines l'2∼l'8 plus l9∼l16.