A transmission capacity of a transmitter is controlled by a signal input from a receiver through a propagation path. The receiver produces a control signal for controlling a system at the optimum transmission capacity which is obtained from monitored information such as transmission quality or reception C/N and the like, and outputs the control signal to the transmitter and a judging circuit. The judging circuit compares the transmission capacity of each line. When the transmission capacity of a first regular line is the lowest transmission capacity, a switching signal is output from the judging circuit. Then a transmission switching device and a reception switching device operate and the input signal on the first regular line is output through the spare line. The transmission capacity of the signal is that of the spare line.