Abstract: An user authentication system for authenticating a user using an IC card in conjunction with a portable terminal used to generate a one-time password and a server used to generate a corresponding one-time password for user authentication. The IC card contains a secret key for generating a one-time password and predetermined random numbers. The portable terminal contains a card receiver for receiving the IC card, a random number memory for reading and storing, and then deleting the random numbers of the IC card, a first password generator for generating a one-time password by the secret key of the IC card and the random number, a first random number changer for changing the random number stored in the random number memory into a predetermined value and storing the changed value in the random number storing portion, and a display for displaying the processed results of the terminal and the server.