At the time of installing an installation-targeted OS in a computer, a supporting tool in a supporting medium carries out the following operations on the supporting tool OS. The operations include: storing information in a response file; copying a unique device driver and a DOS system file from the supporting medium to a hard disk; copying an installation file from the installation medium to the hard disk; and updating the installation file with a security patch file downloaded from the download server through a network. Thereafter, an installer is booted up from the DOS system file to install an OS by use of the response file, the unique device driver, and the installation file.