PURPOSE: To improve the quality of line and to economize the system, by attaining the optimization of a level diagram as the entire communication system, in the satellite communication amplifying two waves with one repeater.

CONSTITUTION: A voltage representing the S/N ratio of a transmission line of an opposite station and that of an own station are applied to a differential amplifier 14, where a subtraction is performed. Further, a prescribed voltage ΔV is subtracted by a differential amplifier 15. After the result is integrated at an integrator 16, it is applied to a variable attenuator 9 of the output of a modulator and a transmission power from a large power amplifier 11 is limited by varying an output to a frequency converter 10 by the attenuator 9. Thus, since the S/N value of the transmission line of the own station is slightly larger than that of the transmission line of the opposite station by setting the prescribed voltage ΔV, there by optimizing the quality of the bidirectional line.