An OOB sequence monitoring unit detects that an OOB sequence carried out between a base device as a superior device and a connection I/F which operates even if an extension device is in a standby state has proceeded to a given stage. Based on the detection by the OOB sequence monitoring unit, a power supply control unit instructs a starting power supply unit to supply power. When the extension device starts, the OOB sequence is carried out between the extension device and the connection I/F of another extension device in the same manner. As a result, extension devices are started in decreasing order from the extension device closest to the superior device.