PURPOSE: To extract target information at much higher speed by preserving temporarily restored information and using this information with respect to the data restoring method for restoring information from a data group hierarchically holding differential data.

CONSTITUTION: Nodes 11 are hierarchically connected, the differential data are held in the respective nodes 11, the information is repeatedly restored based on the differential data in the respective nodes from the leading node to the arrival at the nodes successively defined as targets, the restored nodes add the restored information are held in the case of restoring the information in the target node, and the restoration to the target node is performed based on these held nodes and restored information at the time of restoring the information in the node.