When a request to unlock a door of one side portion of a vehicle is input, an ID request signal is transmitted from a first transmitting antenna provided in the one side portion, and an alarm signal is transmitted from a second transmitting antenna provided in the other side portion of the vehicle. A portable unit sends back a portable-unit signal in dependence on the ID request signal and alarm signal. When the portable unit receives the first signal and does not receive the second signal, a door-unlocking control part unlocks the door of the one side portion in dependence on the portable-unit signal received by a receiving antenna.