A relay device or a connection cable for relaying a video signal from a transmission device to a reception device includes a control unit for detecting whether the received video signal is an AC signal output or a DC signal output, and a current pulling unit for pulling a predetermined current from the received video signal. If the received video signal is an AC signal output, the control unit is configured to control the current pulling unit to pull the predetermined current. If the received video signal is a DC signal output, the control unit is configured to control the current pulling unit to stop the pulling of current, and to transfer the video signal from the transmission device to the reception device.