A user who intends to install software notifies the sales company of such software of the user'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.