PURPOSE: To prevent double power supply to a transmission line reliably by connecting a power supply detecting circuit positively with a line to be selected every time when a transmission line is selected.

CONSTITUTION: When power of a detection circuit 3 is turned ON from a local station for the purpose of maintenance under a state where a line being fed with power from a counterpart station is selected, a capacitor C1 or C2 is connected in parallel with a capacitor C3 even if a transistor 5 is turned ON to charge the capacitor C3. Consequently, charges in the capacitor C3 are absorbed quickly to drop the base voltage of the transistor 5 which can thereby be turned OFF. On the other hand, a charge absorbing capacitor corresponding to a line not selected is short-circuited and discharged thus preparing for next selection. Consequently, the detection circuit 3 is reset positively every time when the circuit is switched resulting in prevention of double power supply to an objective line.