PURPOSE: To transmit with a single transmission line by executing the bidirectional transmission of a sound signal by a base band with a hybrid coil, executing the amplitude modulation, frequency-modulating and transmitting a video signal with a carrier.

CONSTITUTION: The sound signal from mikes 1 and 21, after mike amplifiers are amplified, is inputted to hybrid coils 4a and 4b and respectively inputted through band pass filters(BPF) 5a and 5b to a transmission line. The video signal output of video cameras 11 and 6, after it is amplitude-modulated, is inputted through high pass filters(HPF) 8a and 8b to a transmission line respectively and the bidirectional transmission can be executed. Thus, since the voice is transmitted by the base band, the bidirectional separation is executed by the hybrid coil, and the video signal is amplitude-modulated, frequency-modulated and transmitted with a carrier, respective signals can be easily separated even when a DC current is also simultaneously transmitted, and can be transmitted with a single transmission line.