PURPOSE: To effectively transmit data in correspondence with the fault conditions of a transmission line by compulsorily stopping the supervisory data transmission of a present station when other station supervisory data reception information is inputted at the time of transmitting present supervisory data and preventing the input of other station supervisory data reception information at the time of subsequent transmission of present station supervisory data.

CONSTITUTION: A timer means 3 operates a timer, and transmits a termination signal showing time-over when time corresponding to a timer control signal elapses to a control means 2. Then, the control means 2 transmits the termination signal to a present station transmission part 4 as a present station supervisory data transmission timing and transmits supervisory data to the transmission line. When other station supervisory data reception information is inputted while supervisory data is transmitted, the control means 2 stops the transmission of supervisory data, transmits a second timer control signal to the timer means 3 by using inputted other station supervisory data reception information and prevents other station supervisory data reception information from being inputted at the time of the subsequent transmission of present station supervisory data. Thus, data can effectively be transmitted in correspondence with the fault conditions of the transmission line.