PURPOSE: To realize the automation while avoiding a big recrease of the cost for the exchange by combining the automatic balancing operation with the program control via the central control unit.

CONSTITUTION: When the subscriber number along with the information of the impedance measurement are thrown into central control unit 26 from input/output device 24, test relay contacl t in line circuit 14 switches subscriber lines 12 and 12' to selective connection unit 20. At the same time, line 12 and 12' are connected to impedance measuring device 34. The impedance measured is transmitted to unit 26 to decide the optimum balancing network value. And the relay selection signal is given to signal distributor 30 to energize, for example, relay N1, and thus relay contact n1 is closed in 2-wire/4-wire converter circuit 16. And balancing network BN1 is connected to hydrid coil HYB.