When a shutdown is detected, an MBR or a backup MBR are read. When the data of the MBR is not identical to the data of the backup MBR, the MBR is copied to the backup MBR. When the backup MBR cannot normally be read or is improper,the MBR is copied to the backup MBR. When the backup MBR cannot normally be read or is improper while the power of the computer (16) is turned on, the MBR is copied to the backup MBR