1. A storage apparatus including a disk device having a storage medium storing data, and a controller unit configured to control the disk device, comprising: a first data volume for which a storage capacity larger than the inherent storage capacity of the storage medium is defined, the first data volume including a first real volume associated with a physical storage area in the storage medium and a virtual volume to which a storage area other than the first real volume is allocated; a second data volume including a second real volume associated with another physical storage area in the storage medium; a pool volume to which a storage area is allocated to store the data involved in access to the storage areas allocated to the virtual volume; a pool volume management table storing entries that associate the storage area allocated the virtual volume with the storage area allocated to the pool volumes; a monitor unit configured to monitor the usage rate of the virtual volume; and a transfer unit configured to transfer, in response to a command from the monitor unit, the data stored in the storage area allocated to the pool volume to the second real volume, based on the entries in the pool volume management table.