COMPUTER SYSTEM

PROBLEM TO BE SOLVED: To provide a computer system having a safe and robust security function capable of surely preventing leakage of information.SOLUTION: Information necessary for booting a computer device 10 is erased from a hard disk device (HDD) 15 and set to a USB key 20. When a power supply is applied, a CPU 17 reads a first MBR of the USB key 20 that is connected to the computer device 10 and stores the first MBR, determines whether or not ID information for comparison in booting included in the first MBR matches ID information for comparison in booting stored in the HDD 15, according to identification program included in the first MBR, and when it matches therewith, boots the OS by referring to an OS booting file stored in the USB key 20.