An automatic adjustment system that includes: a transmission device that includes a constant voltage output unit that outputs a constant-voltage signal to a cable; and a reception device that includes a voltage detecting unit and a control unit. The voltage detecting unit receives the constant-voltage signal transmitted through the cable and detects the voltage of the signal, and the control unit determines signal attenuation based on the voltage detected by the voltage detecting unit and adjusts the gain of a receiving unit that receives the signal from the transmission device. This automatic adjustment system performs gain adjustment on the receiving unit when a cable insertion/pull-out sensing unit senses that the cable is connected to a connector.