An OS starting method of an OS stored in a hard disk of an information processing apparatus includes: starting a BIOS stored in a storage when it is determined that a power is turned on; searching for a boot device that can be started; starting a boot OS stored in an external storage medium; displaying a password input screen on a display section; generating an unlock password for the hard disk by concatenating information unique to the external storage medium with an input password and converting the information being concatenated with the input password to a hash value; unlocking a locked state of the hard disk with the unlock password when it is determined that a setting of security of the hard disk is performed; starting a MBR of the hard disk whose locked state is unlocked; starting a boot loader of the OS; and starting the OS.