PURPOSE: To prevent a subscriber circuit efficiently by connecting a 4-line type trunk circuit through a call switch to a measuring subscriber circuit which is equipped with a termination circuit, connecting an oscillator and a receiver through a jack for measuring instrument and measuring a return loss.

CONSTITUTION: When a command is inputted form a teletypewriter 7, according to the instruction of an exchange control circuit 6 to control a call switch 3 of an exchange, a subscriber circuit 2 equipped with a termination circuit 1 is connected with a 4-line type trunk circuit 5 equipped with a jack 8 for measuring instrument to connect the oscillator and receiver. Next, a signal is outputted form the oscillator, which is connected to the jack 8 for measuring instrument in one side, and a returned signal part is received from the subscriber circuit 2 to the receiver in the other side. Then, the return loss is measured. Thus, the subscriber circuit is not vainly used and without necessity for correcting the loss, the return loss of the subscriber circuit can be measured.