A storage controller of the present invention is capable of providing a plurality of external volumes to a mainframe as a single virtual volume without lowering write performance. A virtual volume inside a main storage apparatus is associated with a plurality of external volumes inside an external storage apparatus. When the mainframe formats the virtual volume, a control information creation unit of the main storage apparatus creates and stores control information related to the virtual volume. Consequently, it is possible to enhance processing performance when the write size specified by the mainframe coincides with the data size set in the write destination, that is, during a so-called isometric write.