This invention aims to improve the data guarantee performance of an external storage apparatus. Upon managing a storage extent of the external storage apparatus as a volume, a guarantee code corresponding to read or write access-target data is created with a guarantee code creation/confirmation unit, the created guarantee code is stored in the external storage apparatus, and, upon executing the read or write access to the storage extent of the external storage apparatus, the concordance of the guarantee code stored in correspondence with the access-target data and the guarantee code created from the access-target data is confirmed with the guarantee code creation/confirmation unit 206, whereby the access-target data can be guaranteed.