Whether interruption is present in the connections of automatic door constituting units is detected.A plurality of automatic door constituting units (4, 8, 10, 12, 20, 24 and 26) are interconnected in such a manner that they can communicate with each other via a bus (2). The automatic door constituting units include a door controller (4) for controlling opening and closing of a door. The door controller (4) successively calls the other automatic door constituting units one by one via the bus (2), and the other automatic door constituting units send a response to the calling via the bus (2). The door controller (4) judges that the automatic door constituting unit the door controller (4) called is disconnected from the bus (2), when said called automatic door constituting unit does not send a response within a predetermined time after the calling.