A first storage control apparatus unit has a first logical volume. A second storage control apparatus unit has a second logical volume, which is a virtual logical volume and is mapped to the first logical volume, and a third logical volume that is associated with the first logical volume. The second storage control apparatus unit receives, from a host, an I/O command which has information including, as information denoting an I/O-destination area, an ID of the second logical volume and an address of an area of the second logical volume. The second storage control apparatus unit identifies a logical volume having an area that serves as an entity of the I/O-destination area, and a storage control apparatus unit that has this logical volume.