PURPOSE: To send a signal through plural line type multiplex transmission lines, to provide redundancy and to detect a faulty location even when a short-circuit fault across wires takes place.

CONSTITUTION: On the occurrence of a fault in plural multiplex transmission lines, an error detection means 2 detects a fault by a voltage level of a bus transceiver 3 and information from a transmission control means 1 and a transmission switching signal outputted from the error detection means 2 switches a transmission changeover device 4 and a transmission line connection controller 5 controls individually the connection/opening of the part forming a ring of the plural multiplex transmission lines with a transmission line control signal from the error detection means 2. Thus, part of the plural multiplex transmission lines having a fault is disconnected to attain re-transmission and the double redundancy or over is provided against the fault and the faulty location is detected.