In a computer, a logical partition for calculation in which an OS and an application operate and a logical partition for storage for providing a storage function are constructed. In the logical partition for calculation, a device corresponding to a storage device is provided, while the logical partition for storage provides a volume. A memory space that can be shared by the both logical partitions is prepared, and management information describing a sorting destination or a sorting method of an I/O request issued by an application is provided in the memory. If the logical partition for calculation receives an I/O request from the application, the partition refers to the management information and sorts the I/O request to the storage device or the logical partition for storage. The logical partition for storage processes the received I/O request by the storage function and transmits the result to the storage device.