The present invention suppresses the generation of redundant I/O and improves the response to the host during data migration. When migrating data from the migration source volume to the migration destination volume, the access destination of the host is switched to the second storage device. When data requested by the host has not yet been migrated, data is read from the first storage device. When the frequency of access to the non-migrated area reaches a prescribed value, the target IP address is changed and discovery is implemented so as to switch the access destination of the host to the first storage device. When the frequency of access to the migrated area in the first storage device reaches a prescribed value, the access destination of the host is switched to the second storage device. The access destination of the host is controlled based on the data migration state and access state, and the generation of redundant I/O can be suppressed.