A backup system is configured by interconnecting a backup management device and disk array device via a SAN. The backup management device allows an administrator to set backup cycle, number of backup generations, and save periods. On the basis of these settings, the backup management device performs an overwrite decision process, and executes backup processes in such a way that the administrator can manage the desired number of generations of backup data with a limited number of volumes. By so doing, the administrator is able to manage multiple generations of backup data, while being allowed to set the save period for each generation on an individual basis.