PURPOSE: To eliminate the control of the output level for a monitor signal oscillator when the repeating transmission is set up, by applying the output of a negative feedback amplifying circuit to a hybrid coil and then applying a monitor signal to the hydrid coil via an impedance.

CONSTITUTION: The output of a primary amplifying circuit 1 is applied to the 1st terminal of a hydrid coil 5 and then fed back through the 2nd terminal of the coil 5 via a negative feedback circuit network 2. Then a monitor signal oscillator 3 is connected to the 2nd terminal of the hydrid coil 5 via an impedance 3', and at the same time a balanced circuit network 6 is connected to the 3rd terminal of the coil 5. For the monitor signal level, an output 8 is kept at a fixed level regardless of the quantity of transmission of the circuit 1 and a negative feedback circuit newwork 2 as long as the output level of the oscillator 3 is kept constant. Therefore, an overall monitor is possible for a repeating amplifier by monitoring the output 8.