Each of a portable device and an in-vehicle unit is provided with respective GPS receivers to determine a portable device position and a vehicle position. The portable device transmits a response signal including the portable device position in response to a request signal of the in-vehicle unit. The in-vehicle unit, only in a case where it is determined that the portable device position included in the response signal is within a predetermined area around the vehicle position, performs control of a lock/unlock state of a vehicle door in accordance with the identification result of the ID code included in the response signal. Accordingly, it is surely prevented that the door is remotely controlled by the portable device at far distances from the vehicle.