Provided is a data storing system which holds only updated data by a snapshot action of a disk array subsystem. When there is generated a writing request to snapshot duplicate volumes as the duplicate data storing areas, unused pages for the requested amount is searched from a common logical-volume which is used in common as a real data storage area within the disk array subsystem, and address conversion of the page is performed. The result of the address conversion is held in a directory which is provided for each of the snapshot duplicate volumes. When making an access to the page thereinafter, the directory is always being referred to, and the address conversion is performed from the page of the snapshot duplicate volumes to the corresponding page of the common logical-volume.