PURPOSE: To obtain an economical control system by superimposing a DC voltage on a transmission burst signal from a line terminator, transmitting the result and allowing a radio transmission/reception device to detect to prevent interference/disturbance due to mis-transmission with a simple circuit while using a high frequency.

CONSTITUTION: A base band signal received and demodulated by a reception section 21 of a radio transmitting/receiving device 20 is transmitted to a reception rerminating section 11 of a line terminator 10 via a transmission line, a reference pulse included in a reception signal is extracted and fed to a timing generating section 13. A DC superimposing section 16 superimposes a DC voltage on a transmission burst control signal 100 from a generating section 13 and transmits the result to a DC separating section 28. A DC detecting section 29 separates/detects respectively the DC component superimposed. A switch control section 26 transmits a control pulse 103 based on a confirming signal 101 from the detection section 29, controls a transmission switch 23 to pass an output of a transmission section 22 only for the period of a timing pulse to transmit a radio wave.