PURPOSE: To execute signal transmission with a backup power source without generating a trouble by changing over a line transfer switch provided with a mechanical contact even at the time of the power failure of a commercial power source which operates the transmission/reception terminal equipment of duplicated lines and their interfaces.

CONSTITUTION: It is assumed that the power failure of the commercial power sources 3, 4 or 1, 2 which operate transmission terminals 20A, 20B and their interfaces 21A, 21B of the duplicated lines A, B and the reception terminal equip ment 10A, 10B and 11A, 11B occurs. At this time, a detector 2 detects the power failure, and the four switches of a power source switch are turned on with a detection signal (d). Then, the battery 30 of a spare power source is supplied to the interfaces 11A, 11B and 21A, 21B, and the electrical interfaces 11A, 11B and 21A, 21B are operated, and a transmission signal is transmitted/received. Also, when the power failure occurs in the commercial power source, the com mon contact of the line transfer switch is always brought into contact with one side, therefore, the signal of either the interfaces of the transmission and reception terminal equipment can be always transmitted and received, which prevents the interruption of the transmission from occurring.