PURPOSE: To detect stably the push of an earth button by means of a subscriber circuit which holds tthe current value at a fixed level at the side of the line A and regardless of the length of the line, by switching the voltage monitor to the current monitor at the side of the line B for the push of the earth button.

CONSTITUTION: The current is converted into the voltage by a microresistance 204 and an operational amplifier 205, and this voltage is compared with the reference value 206 by a comparator 207. At the same time, photocouplers 208 and 209 are provided. The output of the 209 is set at L level while a current is flowing toward a line A. The output of the 207 is set at L level when the current of a line B is less than the value 206, and the output 211 is set at H level. Thus it is considered that an earth button is pushed. Therefore the push of the earh button is discriminated if the current flowing to the line B is set at ≤1/2 current value of the line A since the resistance value is generally equal between resistances 106 and 107 when the line has a long length.