PURPOSE: To reduce cost and to make mounting area small in a circuit that makes network control directly connecting to a telephone circuit by replacing an inductance with a transistor, a capacitor and a resistance.

CONSTITUTION: When a relay contact 9 is closed to make communication, DC loop current from an exchange flows from terminals 14, 15 to a DC circuit. This is rectified by diodes 10∼13 and let flow into a resistance 7 and the collec tor of a transistor (TR) 5. At this time, as output of a low-pass filter consisting of the resistance 7 and a capacitor 8 is connected to the base of the TR5, change of AC voltage superposed on Dc loop current hardly gives influences on voltage at both ends of the capacitor 8. Accordingly, the change of base current is small, and the corrector current becomes nearly a fixed value and returns to the exchange. Resultingly, this acts to let flow only DC from current including DC and AC, and accordingly has the same property as an inductor.