A storage controller of the present invention writes data to a storage device, in which the storage unit is fixed, at a size that is larger than this storage unit, and curbs response performance degradation. A host sends write-data in a prescribed number of logical blocks in accordance with a basic I/O size defined at initialization. A controller respectively creates a guarantee code for each logical block, and appends same to the write-data. Write-data, to which a guarantee code has been appended, is stored in another prescribed number of logical blocks in accordance with a basic disk access size which is set at a value corresponding to the basic I/O size, and sent to a storage device. When an unused part is also stored in the storage device, the utilization efficiency of the storage area decreases, but the need to read out data located before and after data targeted for updating at data write is eliminated, thereby curbing the degradation of response performance.