Keyless entry includes a plurality of mobile devices each capable of fitting thereto a transponder for immobilizer authentication and a car-mounted device having a mobile device communication portion for communicating with the mobile devices and an immobilizer authentication portion for executing immobilizer authentication by making communication with the transponders. The immobilizer authentication portion includes a transmission/reception portion for communicating with the transponders, an ID preservation portion for storing and preserving IDs of transponders that have already been registered and a combination ID preservation portion for storing and preserving combinations of IDs of the registered mobile devices and IDs of the transponders fitted to the mobile devices. When executing immobilizer authentication, the immobilizer authentication portion extracts the combination of the ID of the mobile device lastly making communication and the ID of the transponder from the combination ID preservation portion, and executes first the authentication process for the transponder having the ID so extracted.
