A plurality of control groups (8A, 8B, ---) are provided which comprise outdoor control units (5) and indoor control units (6) connected to each other by self-system communication lines (7) so as to enable transmission and reception of signals. The respective control units (5, 5, ---) are connected to one another by an integrated line (10) so as to enable transmission and reception. In the respective control units (5, 5, ---), the self-system communication lines (7) are connected to the integrated line (10) to constitute a single transmission network (NW). Further, the transmission network (NW) is connected to a centralized controller (9) which serves to perform centralized control of outdoor units (3, 3, ---) and indoor units (4, 4, ---). Also, relays (28a, 28b) are provided which serve to establish and cut off continuity between the self-system communication lines (7) and the integrated line (10).