PURPOSE: To decrease the number of control times of a communication controller and to attain the efficient system constitution with less malfunction, by connecting the selected communication controller with a modulation and demodulation device, and making folding connection to the non-selection side automatically.

CONSTITUTION: A line switching device 4 is provided between two systems of communcation controllers 1, 2 and one system of modulation and demodulation device 3, and the switch is provided in the folding direction normally. When a system constitution control signal of the 0 system selection is incoming out of the devices 1, 2, the 0 system commuication controller 1 is selected. Simultaneously, a signal is stored in a tip differentiation circuit 5 to operate a changeover switch SW0 corresponding to the system for the connection of the controller 1 and the MODEM3. In this case, the device 2 is not connected to the MODEM3 while a changeover switch SW1 of the device 4 is kept inoperative, allowing to perform the folding test of all the signals.