A communications network system capable of reducing the time from the occurrence of a trouble to the rebuilding of the system and a rebuilding method thereof. In a communications network system which comprises a plurality of node devices (10-1 to 10-4) distributed at a plurality of positions and transmission lines (1a, 1b) for connecting the plurality of node devices, one of the plurality of node devices is set as a master station (10-1) to operate the system, when the master station has a trouble, the individual node device makes an inquiry to the other node devices, and a node device which has confirmed that no other node device has a higher priority than itself becomes a substitute master station to take the place of the troubled master station to continue the operation of the system.