A virtual LU astride a plurality of storage devices is a virtual volume composed of a plurality of partial volumes. Functions of the storage system, Zoning information of a SAN and a host group information of the virtual volume are inherited to a partial volume created for each physical device. When a page granularity data migration is executed, volume or data are migrated to other storage devices without depending on the relationship between a physical port and the physical device. When a host IO request is received when migration is on-going, response processing is performed by synchronizing storage devices within the range of migration data. The host IO request related to the page being migrated sends a partial complete response including information for re-routing the request to a path of the appropriate data migration destination device capable of issuing the IO.