PURPOSE: To measure the insulation resistance between a subscriber line and the gound by inserting an SCR into a DC feeding circuit of a four-wire terminal device and controlling its conduction on an exchange side.

CONSTITUTION: A tertiary winding 104 is provided to the repeating coil 103 of the transmitting circuit 3 of the four-wire terminal device 10 and the output of the winding 104 is connected to the gate of the SCR 107 provided to the feeding circuit through a rectifying circuit 105. When an AC signal is sent out of the signal generating circuit 202 of an exchange 20, the SCR 107 turns on to close the circuit 101. Further, when a breaking contact SW inserted on the ground side of the exchange 20 is opened, the SCR 107 turns off to open the circuit 101. Consequently, an independent closed circuit is composed of the gound, an insulatting resistance 31, the transmitting circuit 3, the made contacts ta, tb of a testing device 22, terminals TA and TB, a terminal B, an ammeter 27, a battery 25, and the ground 26 to take a measurement of the insulation resistance 31.