A NAS system including a NAS controller which processes file level input/output requests and controls the NAS system, and a storage apparatus having a controller which controls a storage device upon which plural volumes for storing data are represented. In the invention when at least a portion of data of a file stored on a volume is updated, the updated data is stored to an unused area of the volume, information is stored on the volume indicating that the updated data corresponding to original data stored in an original area is stored in the unused area so that subsequent accesses to the original of the updated data is directed to the updated data stored in the unused area, and the original of the updated data is retained in the original area, thereby implementing long term data archiving of the original of the updated data.