An authentication system includes an authentication unit that performs an authentication process between a first communication device and a second communication device when the first communication device and the second communication device communicate. The authentication unit performs the authentication process by transmitting authentication information from one of the first communication device and the second communication device to the other one, calculating the authentication information with an encryption code in each of the first communication device and the second communication device, and evaluating a calculation result. During a processing series in the authentication process, the authentication unit performs a first authentication based on part of the calculation result transmitted between the first and second communication device, and a second authentication based on another part of the calculation result transmitted between the first and second communication device.