When a vehicle door locking operation is conducted, an in-vehicle device checks the vehicle compartment to determine whether a mobile unit exists in the vehicle compartment. When the in-vehicle device receives a response from the mobile unit indicating its presence in the compartment, the in-vehicle device receives a response from the mobile unit, prohibits the door from locking, and activates a buzzer. The mobile unit thus can be prevented from being locked in the vehicle compartment. When the mobile unit receives the search signal, it sounds its buzzer, thereby making it possible to further indicate its location within the compartment.