To reduce the cost by simplifying the configuration of the multiplex system transmitter.

Each transmitter 21 in the active system makes logical judgment based on a received external condition to generate and output a transmission signal and detects whether or not a fault takes place based on the voltage and the current of the outputted transmission signal. When any transmitter 21 in the active system is faulty, a transmitter 1 in the standby system receives the external condition that is received by the faulty transmitter 21 and generates and outputs the transmission signal in place of the transmitter 21. A changeover section 4 selects the transmission signal from the transmitter 21 in the active system or the transmission signal from the transmitter 31 in the standby system depending whether or not the transmitter 21 in the active system is faulty and provides the output of the selected transmission signal to a track circuit.