A first virtual storage and a second virtual storage share an external LU (Logical Unit) inside an external storage. The first virtual storage comprises a first LU, which comprises multiple first virtual areas and conforms to thin provisioning, and an external capacity pool, which is a storage area based on the external LU, and which is partitioned into multiple external pages, which are sub-storage areas. The second virtual storage comprises a second LU, which comprises multiple second virtual areas and conforms to thin provisioning. In a data migration from the first LU to the second LU, for a data migration from a first virtual area, to which an external page has been allocated, to a second virtual area, the first virtual storage notifies the second virtual storage of a migration-source address, which is an address of the first virtual area, and an external address, which is an address of the external page that has been allocated to this virtual area, and the second virtual storage stores a corresponding relationship between the notified migration-source address and external address.