To efficiently convert a station power to supply it to a telephone set circuit.

A public telephone set 4 consisting of a DSU (line terminating device) circuit 11 which is connected to an exchange 1A through a digital line 3 to terminate the digital line and a public telephone set circuit 20 which is connected to the DSU circuit to perform the functional operation of public telephone is provided with a DC/DC converter 12 which converts the station power from a feed part 2 of the exchange 1A to a power having 5V DC voltage at the time of input of this station power through the digital line and supplies it to the DSU circuit and the public telephone set circuit in common.