Fault tolerance is improved, a functional limitation at the time of start-up of an I/O system is avoided, and a start-up time is shortened. A downstream PCI Express bridge sets a PCI Express device connected to the downstream PCI Express bridge itself, among a plurality of single root-compatible PCI Express devices shared by a plurality of root complexes connected to a plurality of upstream PCI Express bridges that exchange data with the downstream PC Express bridge itself through a network, controls and monitors a state of a physical link with the PCI Express device connected to the downstream PCI Express bridge itself, and performs monitoring and notification of an error of the PCI Express device connected to the downstream PCI Express bridge itself.