PURPOSE: To detect a trouble of the data transmission in a short time without increasing the load of a CPU by making each node send a frame to identify its own station at each prescribed time necessary for detection of the trouble.

CONSTITUTION: Each node transmits a frame to identify its own station at each prescribed time necessary for detection of troubles. At least one of these nodes detects the trouble of the node or a signal transmission line in response to the frame received from each node. That is, at least one of nodes A-E connected to a signal. transmission line 10 receives the frames transmitted periodically from the nodes B-E within a prescribed time and detects the trouble of the data transmission according to the reception or non-reception of frames. Thus each of nodes A-E can transmit the frame in a time cycle shorter than the prescribed time set for detection of the trouble. As a result, the congestion of a communication circuit can be prevented together with the increase of the load of a CPU 11.