Abstract: A transmission failure diagnosis apparatus for use in a multiplex transmission system which sends and receives data by time division communication among a plurality of nodes connected by a common transmission path. This transmission failure diagnosis apparatus comprises a reception circuit, connected in the transmission path, for receiving a transmission signal on the transmission path, an amplitude detection circuit for detecting the signal amplitude of a transmission signal received by this reception circuit, and a judgment circuit for judging failures in the system on the basis of the amplitude of the transmission signal detected by the amplitude detection circuit.