Erroneous deletion of data due to a collision of digest information during data de-duplication using digest information is prevented. When backup data is stored on a backup server 1100, digest information of the backup data is generated and stored in a digest information management table 4200. In addition, when a backup data storage request is made to the backup server 1100, a digest information verification control sub-program 1127 generates digest information of data to be backed up, and performs verification against the digest information of the backed up data already stored on the backup server 1100. If, by this verification, it is found that backed up data having the same digest information is already stored, de-duplication is realized by reusing the existing backed up data without newly storing the data to be backed up.