When a door sensor is touched, a vehicle-mounted unit and a portable unit performs mutual communications for authentication, and the doors are unlocked. When the door sensor is touched without intending to unlock the doors, wasteful consumption of electric energy is avoided which would otherwise result from mutual communications for authentication triggered by a request signal. When the door sensor may possibly be operated not for the purpose of unlocking the doors, e.g., when a CPU detects the doors as being already unlocked from a signal outputted from a door lock knob switch, a request signal is inhibited from being transmitted from the vehicle-mounted unit to the portable unit.