A first transmitter outputs a request signal requesting a response to a mobile device toward an area including a driver's seat and its neighborhood, and a second transmitter outputs this request signal toward an area covering the entire vehicle. The mobile device transmits a response signal to a receiver if receiving the request signal transmitted from the first or second transmitter. The controller detects the position of the mobile device based on a communication result between the mobile device and the receiver in response to the request signals transmitted from the first and second transmitters.