Data is stored to a disk drive (4) in a directory path designated by a write request, and a directory path as a backup object and the directory path for the write request are compared with each other. When the directory paths match each other, the data is transferred to backup systems (3 and 5). In the case where the directory path for the write request is not the backup object, the data in the directory path is requested to be backed up when the directory path matches a preset condition.