PURPOSE: To supervise an entire transmission line with a division by both terminal stations at the same time by adopting a constitution so that a repeater does not send a transmission signal from the 2nd terminal station to the 1st terminal station when the 1st terminal station sends a command signal to the repeater.

CONSTITUTION: When the 1st terminal station 1 sends the command signal to the repeater R1, the repeater Rn does not send the transmission signal of the 2nd terminal station 2 from the repeater Ri to the 1st terminal station 1. Thus, the 1st terminal station 1 needs not stop the transmission of the transmission signal to the 2nd terminal station 2 and sends the command signal to the repeater Ri so as to receive the transmission signal sent from its own station at the repeater Ri foldingly thereby attaining the supervision of loop back. While the 1st terminal station 1 applies the supervision of loop back at the repeater Ri, the 2nd terminal station 2 applies the supervision of loop back by allowing the repeater Ri to fold the sent signal or allowing to the repeater Rn closer to the 2nd terminal station 2 than the repeater Ri to fold the signal similarly. Thus, the supervision of loop back is applied from one direction and also from both the directions.