First and second codes specific to each program are prepared. The first code is encrypted to generate a first encrypted code that encrypts the second code to generate a second encrypted code. The second encrypted code is stored in a memory medium. An install key that the user needs to input is made by combining the first code and the second code. To authorize installation, the first code contained in the input install key is converted into the first encrypted code in the medium, and a computer on which a program is installed obtains the first encrypted code and generates the second encrypted code by encrypting the second code contained in the install key on the basis of the first encrypted code. Then, the computer reads out the second encrypted code stored in the medium and compares both of them, thereby giving the installation authorization.