PURPOSE: To efficiently use a line by continuously and successively checking the presence/absence of any system abnormality at building unit managing devices in a certain order previously decided among the building unit managing devices.

CONSTITUTION: When the system abnormality is generated at a building unit managing device 25, for example, no response comes from the building unit managing device 25 even when a system abnormality presence-/absence check signal 6 is sent from a preceding building unit managing device 24, therefore, the building unit managing device 24 judges the system abnormality is generated at the next building unit managing device 25, and a system abnormality presence alarm part transmits a signal 7 through a transmission control part to a building group managing device 1 so as to show the system abnormality is generated at the next building unit managing device 25 allocated the present machine. At the same time, a complement system abnormality presence/absence check transmission part transmits the system abnormality presence/absence check signal 6 further to the next building unit managing device 26 according to the prescribed check order.