PURPOSE: To permit a monitoring station to receive monitoring data which is sent from a plurarity of stations to be monitored using the only one-direction transmission line by sharing one monitoring signal carrier wave with a plurarity of stations to be monitored and to shorten the time for transmission and reception of monitoring data by permitting the only station to be monitored whose monitoring information is changed to send the data.

CONSTITUTION: The monitoring signal carrier wave with the station name code of all stations to be monitored R1, R2,...Rn added is transmitted from a remote terminal station Rn to a monitoring station M. In stations R1 and R2 to be monitored, the station name code of the self station is detected, and the monitoring data of the self station is newly added after the self station code to be transmitted to the monitoring station M again. The second invention is that the monitoring data is added to the monitoring signal carrier wave only at the change of monitoring information in each station to be monitored.