PURPOSE: To simply and efficiently conduct the system test by allowing a reception side to form automatically a loopback circuit in receiving the loopback signal from the sender side so as to send back a test signal to the sender side.

CONSTITUTION: An in-band signal device 20 at the sender side closes a switch 10 to send a loopback signal having a frequency different from that of a signal current from the oscillation circuit 1 through a transmission circuit 2 together with a test signal. An in-band signal device 30 at the receiver side branches a reception signal inputted to a terminal LINER into two. The one signal is inputted to a reception circuit 4 as it is and the other signal is subject to only loopback signal extraction by a band pass filter 7 and the result is inputted to a rectifier circuit 6 and converted. The output of the circuit 6 is inputted to a comparator 5 via a time constant circuit comprising a resistor 8 and a capacitor 9. A control output of the comparator 5 is inputted to a switch circuit 3 and the output of the receiver circuit 4 is switched from the position of terminal 4WR to the position of terminal 4WS thereby forming a loopback circuit. Thus, the system test is conducted simply and efficiently.