An in-vehicle device notifies a mobile device of a frequency channel which has been stored in its memory unit, and sets the notified channel to a reception channel. The mobile device transmits a reply signal through the channel notified from the in-vehicle device. When the in-vehicle device determines that a code included in the received reply signal is identical with a code specific to a vehicle, the in-vehicle device stores the channel used for receiving the reply signal in its memory unit. As a result, each communication is conducted through the same channel, which enabled the successful communication previously, thereby enhancing the certainty of communication.