PURPOSE: To erase a comparing circuit from a subscriber circuit, to simplify a subscriber circuit and to decrease a cost by measuring the line resistance of respective subscribers with a test pull-in function and changing over the supply voltage of a DC current to a subscriber based on a measuring result.

CONSTITUTION: By the instruction of a control device 5, a testing device 4 operates a test pull-in relay test of respective subscriber circuits 1 and measures the resistance of lines La and Lb of respective subscribers. The device 5 compares a resistance measuring result RL with a resistance value RLTH set beforehand. Based on the result, in case of RL<RLTH, a ground G selecting signal is sent to respective subscriber circuits 1 and in case of RL≥RLTH, a booster power source +50V selecting signal is sent to respective subscriber circuits 1. The circuit 1 receives a G/+50V selecting signal SEL and transfers a selecting signal SEL to a changing-over circuit 3. The changing-over circuit 3 connects a plus side feeder (b) and the ground G when a selecting signal SEL is a G selecting signal and connects the plus side feeder (b) and the booster power source +50V when the signal is the +50V selecting signal.