In a system to which a plurality of devices are connected, each device has heretofore required a connector for an AC adapter and a connector for a signal line. Further, in order to arrange it so that the amount of power supplied to the overall system will not exceed a limit value, it has been required to adjust the amount of power during system use by turning the power source of each device on or off. According to the present invention, a signal line for data transfer and a power line for supply of power in a system connecting a plurality of devices are consolidated in a single cable to make it possible to connect both lines to one connector of each device. The power controller of a certain device acquires the properties of a plurality of devices connected to this device, decides the optimum power distribution and controls the power controllers of each of the other devices.