PURPOSE: To realize the communication branch equipment in an HDLC system remote supervisory controller with high reliability in which no data missing takes place even when an outgoing data are monitored.

CONSTITUTION: The system is provided with a host level control station 1 implementing monitor of incoming data and control of outgoing data, a subordinate level control station 2 implementing monitor of incoming data only, transmission control sections 26, 25 used for both the stations, and a reply control section 28 implementing an acknowledge to a controlled station 3 when both the control stations 1, 2 receive data correctly depending on the state of the sections 26, 25. Thus, when the incoming data from the controlled station 3 are sent to both the control stations 1, 2 and a transmission error takes place in any of the control stations 1, 2, the re-transmission of data from any of the control stations 1, 2 is attained, a reply frame corresponding to the data whose transmission confirmation is finished at both the control stations 1, 2 is sent to the controlled station 3.