A backup method for a system including a network attached storage including a primary volume and a secondary volume, and a backup server connected to the network. The network attached storage performs a resynchronization process when a backup request is received at a time point that consistency of a file system can be guaranteed, the resynchronization process making the contents of the primary and secondary volumes coincide with each other, splits the secondary volume from the primary volume and transfers data of the secondary volume to the backup server while an online operation with the primary volume continues. The backup server stores the transferred data in a recording medium. The network attached storage performs again the resynchronization process for making the contents of the primary and secondary volumes coincide with each other, after backup completion.