To provide a high-security authentication method, authentication system, authentication program and recording medium, preventing spoofing even if an ID or a password is leaked.

In this authentication method, when connected from portable equipment 20, the ID and the password are inputted to an authentication device 10. When the ID and the password accord with a user ID and a password inside a database, an encrypted code is transmitted to the portable equipment 20 from the authentication device 10. Because the portable equipment 20 has a decryption key paired with an encryption key of the authentication equipment 10, the portable equipment 20 decrypts the encrypted code, and further transmits the decrypted code to the authentication device. Thus, only when the decrypted code transmitted from the portable equipment 20 and the code encrypted and transmitted from the authentication device 10 side match, a person is regarded as the legitimate user, and the connection is permitted.