PURPOSE: To accommodate a relay line with a common circuit and to simplify an exchange by selecting a common line signal system or a simple common line signal system corresponding to a common line signal or a simple common line signal identified by a common line signal reception control section.

CONSTITUTION: When the system is set to the common line signal system, a processing control circuit 201 generates a common signal from a received control signal 701 and sends the result as a signal 301. When the system is set to the simple common line signal system, a simple common line signal 401 received from a trunk section 801 via a time division switch 101 is converted into the signal 301 and sent. The trunk section 801 starts the operation by receiving a simple common line signal 404 and the simple common line signal is sent to a relay line via a time division switch 101 and a common line signal transmission control circuit 501 to accommodate the common line signal or the relay line and the simple common line signal with the common circuit.