1. A storage apparatus, comprising: a storage device having a storage medium storing data; and a controller unit configured to manage a plurality of first type data volumes, a second type data volume and pool management information, and to control transfer of data to the storage device, wherein each volume of the plurality of first type data volumes provides a storage capacity, which is larger than the inherent storage capacity of a first physical storage area in the storage medium, to a host computer, each of the first type data volumes including a first real volume and a virtual volume, wherein the first real volume is already allocated to the first physical storage area in the storage medium before receiving a data write access to the first real volume included in one of the plurality of first type data volumes, wherein the virtual volume is not yet allocated to a second physical storage area before receiving a data write access to a virtual storage area of the virtual volume included in the first data volume included in the one of the plurality of first type data volumes, and the virtual volume is dynamically allocated to the second physical storage area achieved through a pool volume commonly used by the plurality of first type data volumes, when the data write access to the virtual storage area of the virtual volume included in the one of the plurality of first type data volumes is received, wherein the second type data volume provides a second type storage capacity, which is equal to the inherent storage capacity of a third physical storage area in the storage medium, to the host computer, and is a second real volume already allocated to the third physical storage area in the storage medium before receiving a data write access to the second type data volume, wherein the pool management information includes a plurality of sets of mapping information each mapping the virtual storage area of the virtual volume included in the one of the plurality of first type data volumes to the second physical storage area, which is achieved through the pool volume and is allocated to the virtual storage area of the virtual volume included in the one of the plurality of first type data volumes, and wherein the controller unit is configured to monitor a usage rate of the virtual volume of each of the plurality of first type data volumes, the controller unit is configured to, if the usage rate of the virtual volumes of the first one of the plurality of first type data volumes is over a threshold, migrate data from the second physical storage area, as allocated to the virtual volume of the first one of the plurality of first type data volumes, to the second type data volume, and the controller unit is configured to delete one set of the mapping information, corresponding to the virtual volume of the first one of the plurality of first type data volumes and included in the pool management information, to reduce the number of set in the plurality of sets of mapping information in the pool management information.