Port management information is prepared for managing information related to the status of each of a plurality of ports possessed by a storage system in a unified manner. Change of the status related to any one of a plurality of ports from “normal” to anomalous is detected. For a subject external device which is using the anomalous path as an I/O path, a “normal” port is selected, on the basis of the port management information, from one or more ports related to one or more paths which are being used as alternate paths. And information related to this “normal” port which has been selected is notified to the subject external device. During path changeover, the subject external device selects as an I/O path an alternate path which is related to the “normal” port specified from the notified information.