An on-board apparatus control system comprising a portable device and a vehicle-mounted device is provided. The vehicle-mounted device controls an on-board apparatus by determining whether or not the portable device exists in an authorized area. The vehicle-mounted device causes separate transmission antennas to transmit a request signal and a determination signal, respectively. Responding to the request signal, the portable device transmits a response signal that includes information on a received strength of the determination signal. When the received signal strength of the determination signal is greater than a specified value, the vehicle-mounted device controls the on-board apparatus on assumption that the portable device exists in an unauthorized area even when the response signal is received.