A virtual volume control unit allocates, upon detection of a write request for new data in a virtual volume to be accessed, an actual storage space of a physical medium existing in the same storage group to the virtual volume in accordance with volume capacity of the new data. A storage control unit stores the new data as actual data in the actual storage space of the physical medium allocated to the virtual volume. And a restoration control unit causes, upon detection of a fallback in data redundancy in the storage group, the actual data in a stored space among the actual storage spaces in the physical medium that caused the fallback to be preferentially restored in an actual storage space in a destination physical medium with reference to the physical media other than the physical medium that caused the fallback in the storage group.