A control unit having an immobilizer function includes a circuit for transmitting a request signal within a predetermined range. After receiving a reply signal from an electronic key in response to this request signal once and allowing a movable body to move, when a reply signal to the periodically sent request signal is not received, it is determined that the electronic key is out of the range, and it is assumed that the key has been dropped. A monitor section outputs a warning signal and transmits the warning signal to the navigation unit. A navigation unit, in response to the warning signal, stores the location at that time as the positional information on the electronic key. The navigation unit also displays a map showing the location of a key based on positional information on the electronic key. With this system, a lost electronic key may be readily located.