An authentication method is used by an automated driving system that includes a vehicle and an external device, the external device communicating with the vehicle to cause the vehicle to implement automated driving. The vehicle holds a first certificate that certifies validity of the vehicle. The external device holds a second certificate that certifies validity of the external device. The authentication method includes: validating a third certificate that certifies validity of a combination of the vehicle and the external device, in accordance with a result of device authentication performed between the vehicle and the external device by reference to the first certificate and the second certificate.