In a data backup method in a network storage system in which mirroring is conducted between two disk units, the mirroring is delayed by accumulating update data in a cache for a predetermined period of time. When storing the update data in the cache, files are related to each other, and information of file open and close is also stored. The update data is reflected in the mirrored disk using the file open and close information. As a result, the contents of the mirrored disk are delayed for a predetermined period of time from those of the primary disk, and all files are in other than the open state. Within a range of the period to store data in the cache, it is possible to restore data created at a point of time preceding a desired period of time. This enables recovery if a user erases files by mistake.