PURPOSE: To surely protect a subscriber's circuit from the overvoltage of a steep negative surge voltage generated in a subscriber's line by providing a capacitor, a diode and a charging circuit for compensating for the delay of operation of a surge arrester.

CONSTITUTION: A charging circuit 38 charges a capacitor 36 up to such a voltage that a diode 35 does not make continuity at the time of usual operation. For that reason, since an inverse bias voltage is applied to the diode 35, so that the cathode terminal comes into a high-impedance state, the capacitor 36 is cut off from a subscriber's line 31, so that the existence of the capacitor 36 does not have influence upon a call. While a negative surge voltage is generated in the subscriber's line 31 and the voltage exceeds the charging voltage of the capacitor 36 and the surge arrester 34 actually operates, the diode 35 comes into the continuity state and a current is supplied from the capacitor 36 to the subscriber's line 31 and a voltage drop is generated in a resistor 33, so that a large negative voltage is not applied to the input of a feeding part 32.