When destinations of memory devices constituting one virtual volume increase, a relay device performs data transfer among plural memory devices such that a structure of the virtual volume can be changed so as to reduce the destinations of the memory devices as much as possible with this increase in the destinations as an opportunity for data transfer. In addition, when memory areas with a relatively small capacity increase among unused memory areas in which a virtual volume is not constituted, the relay device performs data transfer among the plural memory devices such that the number of the memory areas with a small capacity is reduced as much as possible with this increase in the memory areas as an opportunity for data transfer.