The present invention makes it possible to suitably create additional information on the basis of backup target data and then store the additional information in a data storage medium. In one embodiment, a system comprises a backup destination storage device (7A) that constitutes a backup destination of backup target data to be a target for backup; a backup source device (1), comprising a backup execution module (11) that backs up the backup target data by transferring the backup target data to the backup destination storage device (7A); an additional information storage medium (7P), which is capable of storing additional information created on the basis of the backup target data; and an additional information creation module (21), which creates additional information on the basis of the backup target data and stores the additional information in the additional information storage medium (7P) after a backup of the backup target data has been completed.