To make a secure telephone call even when a public telephone exchange which operates with a station power source is arranged at a distance from an exchange and a station source voltage drops.

In the public telephone set equipped with a telephone set circuit 2, a card reader 4 which conveys an inserted card and records and reproduces information on the card, a battery BAT which supplies electric power to the card reader, a DC/DC converter 13 which inputs the station electric power from lines L1 and L2 and feed it to the telephone set circuit such as a speaking circuit, and a charging circuit 14 which charges the battery, the output of the converter 13 is connected to the input of the charging circuit 14. Further, a changeover switch SW is provided between the converter 13 and charging circuit 14 and when the public telephone set is placed in off-hook state, the converter 13 and charging circuit 14 are disconnected by the switch SW.