If a failure occurs in a certain layer of a conventional hierarchical backup system, the lower layers are also affected by the failure. If, for instance, a failure occurs in the second layer, a backup process cannot be performed in the third layer so that an improper backup results in the fourth layer. When a failure occurs in a certain layer of a hierarchical backup system of the present invention, the influence of the failure is minimized by causing a lower layer to substitute for the layer where the failure exists, by making backups skipping the layer where the failure exists, or by reducing the backup intervals for the lower layers.