PURPOSE: To realize a token pressing ring fault detection system which is capable of quickly specifying a faulty node.

CONSTITUTION: One node of plural nodes for which a cascade connection is annularly performed via a duplexed ring is designated as a system control node 1. Each node is provided with a token pattern loss detection means 11 detecting the loss of a token pattern and a diagnosis request frame transmission means 21 transmitting a diagnosis request frame having at least information designting a self-node when the loss of the token pattern is detected. Therefore, a system control node receives the diagnosis request frame, decodes the frame, recognizes the node detecting the loss of the token pattern and specifies a faulty node based on the recognition result.