Areas are hierarchically ensured as directories for data to be recorded in advance. Therefore directories and files are continuously arranged as much as possible on a disk, and the number of seeks during data read, which is conventionally large because other data is mixed in the same area, is decreased as few as possible. When the files and directories belonging to directory 1 (DIR1) and directory 2 (DIR2) are recorded on a disk and when the files and directories belonging to directory 1 (DIR1) are recorded in an area-ensuring mode, an area on a disk for the files and directories belonging to the directory 1 (DIR1) is ensured previously. The positions where lower-order Files 1, 2 and directories of the directory 1 (DIR1) are recorded in an area concerning the directory 1 (DIR1) as in the case of File 1-1, File 1-2, and File 12. File 3 of the directory 2 (DIR2) is not recorded in continuous areas for the directory 1 (DIR1).