An initiator holds commands corresponding to ORBs issued to a target in an I/O request queue until it receives a completion response from the target. The target has read and write execution agents, and processes commands from the initiator. When a connection between the initiator and target is disconnected, and is connected again, the initiator deletes all ORBs, and generates and issues ORBs again to the target on the basis of the commands held in the I/O request queue. Upon processing an ORB, the target holds an identifier of the ORB whose processing is in progress, and the address of a buffer which is undergoing a read or write. After re-connection, the target compares the held identifier with the identifier of an ORB re-issued by the initiator. If the two identifiers match each other, the target restarts the read or write from the held address. If the two identifiers do not match, the target determines that the corresponding ORB has already been processed, and sends back processing completion status to the initiator.