PURPOSE: To attain connection/disconnection of a power supply without use of power from an external power supply and without a forced interruption circuit by providing a rectifier means, a smoothing DC power storage means, a reverse flow preventing means and an external power supply connection/disconnection means.

CONSTITUTION: A rectifier means rectifies a call signal being an AC component of a telephone line signal 101. A storage means stores an averaged DC power. A DC power from an external device is stored in the storage means via a counter flow preventing means and does not counter-flow to the supply side. A connection/ disconnection means connects an external power supply when the smoothed dC power is a prescribed value or over to keep the connection. When an external DC power supply is stopped and the power inputted to the connection/disconnection means is a prescribed value or below, the external power supply is disconnected. After the input of a call signal 102 is interrupted, when the smoothed DC power of the storage means is decreased and the smoothed DC power to the connection/disconnection means is a prescribed value or below, the connection/disconnection means interrupts the external power supply. Through the action Above, the connection/disconnection of the external power supply is controlled without use of external power supply and of a forced interruption circuit.