To provide an authentication system for improving security, using fewer encryption keys. An authentication requesting device (20) and an authentication target device (10) hold common encryption keys. The authentication requesting device (20) encrypts an authentication code, then selects mutually different portions in the authentication code as switching target portions, and sends an encrypted authentication code and information about the switching target portions to the authentication target device (10). The authentication target device (10) switches the switching target portions in a code obtained by decrypting the received code, to thereby produce a switched authentication code, then encrypts the switched authentication code, and sends to the authentication requesting device (20). The authentication requesting device (20) compares the switched authentication code obtained by decrypting the code received with a code obtained by switching the switching target portions of the authentication code to carry out an authentication process for the authentication target device (10).