A storage system is migrated without stopping service provision by a host computer. By this means, in a migration-source storage system, data of the cache memory is destaged, and, next, data received from the host computer is directly written in a logical unit by bypassing the cache memory. On the other hand, in a migration-destination storage system, communication with the migration-source storage system is performed to set setting information of a logical unit of the migration object into a logical unit management table and set a writing mode for the cache memory to a cache-bypass mode. After that, the migration-source storage system blocks a path to the host computer. The migration-destination storage system receives a report of the path block from the migration-source storage system and then opens a path between the own system and the host computer.