To operate two public telephone sets connected to a digital channel by a station power source by inputting the station power source from an exchange through the digital channel, converting it to first and second power sources, supplying the first power source to a DSU and a first telephone set circuit and supplying a second power source to a second telephone set circuit.

The digital channel 3 is housed in the exchange 1A inside a telephone station 1 and the public telephone set 4A is connected to the U point. When a commercial power source fails and power supply to the public telephone sets 4A and 4B is stopped, the station power source is supplied from a power feeding part 2 inside the exchange 1A. When the station power source is inputted, a DC/DC converter 12 converts it to a 5V voltage and supplies it to a DSU circuit 11 and a public telephone set circuit 20. Also, the DC/DC converter 12 supplies it from a 40V output terminal through a power source line to the DC/DC converter 50 of the public telephone set 4B via a T point and uses it as the power source of the public telephone set circuit 30.