PURPOSE: To shorten the test of a fault contact detecting function within one cycle in a call signal by effectively using the existing constitution elements while excluding the increase of constitution elements.

CONSTITUTION: A call signal impressing circuit 8 which opens a DC voltage supply device 1-side and which selectively impresses the call signal overlapped with DC voltage on a DC voltage supply line and a ground potential supply line is provided for a subscriber circuit having opening/closing elements for terminal equipment detachment 4 and 5, the DV voltage supply line 2, the ground potential supply line 3, a fault contact supervisory voltage extraction circuit 6 and a short detection circuit 7. The call signal overlapped with DC voltage is selectively supplied to the DC voltage supply line or the ground potential supply line from the call signal impressing circuit 8 in a state where the opening/closing elements for terminal equipment detachment 4 and 5 are opened and the DC voltage supply device 1-side is opened.