A bus power-supply device structured to supply power from a power-supply voltage of a node to a serial bus connected to the node through a physical layer and a plurality of connectors conductive to each other of the node, which device, when none of a power-supply voltage of the node is supplied, supplies a DC voltage from the serial bus to the physical layer and when the power-supply voltage is supplied, cuts off a path for supplying a DC voltage from the serial bus to the physical layer to supply a DC voltage from the power-supply voltage to the physical layer.