A transmission system including a transmission device capable of transmitting a video signal, and a reception device capable of receiving the video signal transmitted by the transmission device, the transmission system includes: a processor connected to a memory; a video transmission circuit and a video reception circuit that transmit the video signal between the transmission device and the reception device; a transmission-side data transceiving circuit and a reception-side data transceiving circuit that transmit a data signal between the transmission device and the reception device, wherein the processor is configured to selectively execute a video signal transmission mode in which transmission of the video signal is performed and transmission of the data signal superimposed on the video signal is performed, and a data signal transmission exclusive mode in which transmission of the video signal is stopped and transmission of the data signal is performed.