An authentication system is provided with: a first input unit and a second input unit into which authentication information can be input; a first computing unit which performs computation on the basis of the authentication information input into the first input unit and a communication counterpart-side authentication parameter registered in a communication counterpart; a second computing unit which performs computation on the basis of the authentication information input into the second input unit and a portable terminal-side authentication parameter registered in a portable terminal; and authentication units which, if the authentication information has been input into the first or the second input unit, perform authentication on the basis of the result of computation by corresponding computing unit and the portable terminal-side authentication parameter.