A storage controller having a plurality of storage device and a control circuit providing a plurality of virtual volumes, to each of which a storage area in a plurality of pool volumes provided with the plurality of storage devices can be mapped for writing data in response to a write access sent from an information processing apparatus to a logical area in one of the plurality of virtual volumes, respectively. The control circuit, according to a search of the plurality of pool volumes for a certain storage area in which a certain data pattern is written, release the certain storage area from mapping to a logical area in the plurality of virtual volume, so that the control circuit can use the released certain storage area for mapping to a virtual volume of the plurality of virtual volume as a destination of another write access from the information processing apparatus.