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 a satellite communication amplifying two waves with one repeater.

CONSTITUTION: A voltage representing the S/N value of a transmission line of an opposite station passes through an upper and lower limit circuit 14, then it does not exceed a voltage corresponding to the S/N value of the upper and lower limit determined in advance. A voltage of the S/N value of a transmission line of an own station is compared with the voltage of the circuit 14 and the transmission power of the own station is limited so that the output is increased by ΔV. If the level of a subcarrier is decreased abnormally, a switch 20 is selected to the position (2) with an output of a detector 19 and the control of a variable attenuator 9 is performed with a constant voltage Va. Thus, the level diagram is optimized regardless of either fine or rainy condition.