PURPOSE: To incorporate a transmission line into one system by converting a signal being the result of superimposing a control signal onto a hi-fi modulation signal into an optical signal, transmitting the signal as light, converting the received optical signal into an electric signal, separating the hi-fi modulation signal and the control signal and using the control signal so as to control the synchronizing operation between the transmission side and the reception side.

CONSTITUTION: A signal superimposing circuit 5 superimposes the synchronizing operation control system from a synchronizing operation control signal generator 3 onto the hi-fi modulation signal from a hi-fi modulating circuit 4. An LED drive circuit 6 drives an LED 7 according to an output of the signal superimposing circuit 5 and converts the electric signal into an optical signal. A photodiode 10 converts the optical signal into the electric signal. A low pass filter 12 extracts a synchronizing operation control signal from a signal being the result of the output signal of the photodiode 10 being amplified by an amplification circuit 11 and a high pass filter 16 extracts a hi-fi modulation signal. A system microcomputer 14 controls a DAT mechanism 15 based on the synchronizing operation control signal from a limiter amplifier 13. Thus, the transmission line is unified into one system.