PURPOSE: To take a test in the same state as that in operation by connecting a dedicated trunk to one general trunk with a time-division switch and forming a return loop in the general trunk or on the connection side for the circuit, and taking a return loop test from a common line signal link controller.

CONSTITUTION: A central processor 4 controls the time-division switch 3 to form a loop (c) in the time-division switch 3 to which a test trunk 10 is connected. The central processor 4 controls the time-division switch 3 to set paths (a) and (b), the general trunk 11 and dedicated trunk 2 are connected, and a switch S1 is operated to switch the connection between the trunk 11 and circuit to the connection between the trunk 11 and test trunk 10. A test signal sent out of the common line signal link controller 1 returns to the test trunk 10 through the dedicated trunk 2, paths (a) and (b) in the time-division switch 3, general trunk 11, test trunk 10, and loop (c) in the time-division switch 3 and passes through the route in the opposite direction and is received by the common line link controller 1.