An inexpensive data storage technique utilizing available capacity in individual computer devices connected to a network is provided. When a backup client (BC) (14) of a user PC receives a backup instruction for a file from a user, the backup client (14) requests backup to a backup control server (20). The backup control server (20) divides and encrypts the file to be backed up into a plurality of encrypted pieces, transfers the encrypted pieces to user PCs (10), and stores the encrypted piece in the HDDs (12) of the user PCs (10). When the destributively backed up file is to be extracted, the user PC 10 obtains each encrypted piece from the user PCs 10 on which they are stored, and combines and decrypts the encrypted pieces to restore the original file.