A code range which is the range of usable identification codes (corresponding to file names or directory names) is stored in an area defining area corresponding to a directory, and a layer structure in which the area defining area concerned is set as a parent layer and the other area defining areas are set as child layers is constructed on the basis of the code range. Further, the capacity (empty capacity) of the usable storage area is stored in the area defining area, and the capacity and identification codes which are usable in the layer of an area defining area are restricted by the empty capacity and the code range.