A second storage maps a migration source volume to a virtual volume of a migration destination volume according to storage virtualization technology. A host system including a host switches an access path from an access path to the migration source volume to an access path to the migration destination volume. The second storage executes copy processing of migrating, from the migration source volume to the migration destination volume, data in an assigned area of a virtual volume according to thin provisioning of the migration source volume based on the information contained in the first thin provisioning information in the first storage, and copying that data from the migration destination volume to a virtual volume according to thin provisioning of a copy destination volume in the second storage. The second storage associates the virtual volume of the copy destination volume with the migration destination volume in substitute for the original virtual volume.