In recent years, data life cycle management, in which data is relocated from, for example, a new storage sub-system to an older storage sub-system in accordance with how new the data is or the frequency of use of the data, has become important. One technology for achieving data life cycle management is technology for migrating the contents of a storage area (“volume”) of a storage sub-system to another volume without affecting the host computer that uses the volume. In the present invention, when an associated source volume (for example, the source volume in a copy pair association) of a pair of associated volumes is migrated, migration of an associated destination volume (for example, the target volume in the copy pair association) is also controlled. In this way, it is possible to control the migration of a pair (or a group) of associated volumes in accordance with the user's requirements.