Synchronous signal overlapping and outputting means and sound signal transmitting means of a signal transmitter transmit any color signal which synchronous signals overlapped and the other color signals, and a sound signal on which polarity information of the synchronous signals was added through four signal lines to a signal receiver, respectively. Synchronous signal recovering and outputting means of the signal receiver recovers the synchronous signals on the basis of the synchronous signals separated from the color signal with the polarity information and outputs the signals to an image output unit. Then, it is possible to output sounds together with images inputted from the image inputting source from the image outputting unit with only four signal lines.