An authentication device to be mounted on a vehicle includes a processor. The processor is configured to execute first authentication based on biometric information acquired from a user and biometric information preregistered in the vehicle. The processor is configured to execute second authentication based on identification information acquired from a mobile device of the user and identification information preregistered in the vehicle. The processor is configured to permit start of the vehicle when at least one of the first authentication and the second authentication is successful. The processor is configured to set an authentication function of the first authentication from ON to OFF when at least one of the first authentication and the second authentication is successful.