The present invention makes it possible to change over a volume transparently with respect to a host. A first storage device 2A and a second storage device 2B are connected through a communication network 3B; a volume 8 provided by the second storage device 2B is mapped as a volume of the first storage device 2A. A host 1 is capable of accessing either of the volume 6 or volume 8 via a virtual LU 4. A connection control unit 5, which may be constructed of tables or the like, connects the virtual LU 4 to either one of volume 6 and volume 8. Changeover of the connection target is performed transparently with respect to the host 1 and so is not recognized by the host 1. Consequently, the actual data storage target volume can be changed over while the host 1 always accesses the same virtual LU 4.