PURPOSE: To execute the order wire circuit among a master station, a relay station and a terminal station with only one time slot by inserting an order wire signal for the master station, the relay station and the terminal station in addition to one time slot.

CONSTITUTION: PCM code converters 11, 21, 31 convert reversely an order wire voice communication signal into a PCM digital signal respectively. The digital signal is fed directly to microwave transmitter-receivers 12, 32 of a master station 1 and a terminal station 3 respectively but fed to a microwave transmitter-receiver 23 through a digital arithmetic processing circuit 22 in a relay station 2. On the other hand, each order wire digital signal sent from the master station 1 and the terminal station 3 is inputted to the circuit 22, in which each signal is added and inserted to each other. That is, first, signals from the master station 1 and the terminal station 3 are added and inserted to the digital signal and the result is outputted for the relay station, secondly signals from the relay station 2 and the terminal station 3 are added and inserted to the digital signal and the result is outputted for the master station 1 and thirdly, signals from the relay station 2 and the master station 1 are added and inserted to the digital signal and the result is outputted for the terminal station 3.