a user who intends to install software notifies the sales company of such software of the user &# 39 ; s own credit card information . then , the software sales company communicates such credit card information to the credit company to request the confirmation thereof . if the credit company confirms the user to be the legitimate user , the software sales company produces the installation key uniquely corresponding to the user in accordance with the credit card information for issuing it to the user . when the user inputs the installation key thus issued and the credit card information into an installer . the installer produces an installation key from the credit card information . if the installation key thus produced and the installation key thus inputted are in agreement , the installer executes installation , hence determining the legitimate use of software by avoiding the problems to be encountered in processing software or analyzing it , while coping with the replacement of computers due to out of order or the like .