In order to eliminate I/O performance bottleneck of a path in a disk array system, one expander is composed of two internal expanders connected to each other, and when the path falls in a bottleneck, connection of two internal expanders in a designated expander is cut off, and the cut-off internal expanders are connected to a new path to change access route to the disk drive. A device management table managing individual disk drives and a routing table managing access routes of respective paths are updated according to the switching of the access route.