When vehicle doors have been opened and closed, request signals are transmitted from external transmitters and an internal transmitter to perform mutual communication with a portable device. At this time, when matching of an ID code of the portable device is OK, this matching result is stored. In mutual communication of one of when the vehicle doors are opened and closed, and when door lock switches are operated, if matching of the ID code is OK, the vehicle doors are automatically locked. In this way, by increasing chances of mutual communication between a vehicle unit and the portable device, it becomes more likely that the vehicle doors can be locked.