PURPOSE: To minimize the return loss, by providing the 1st balancing circuit network making an input impedance of a circuit viewed from a two-wire line constant and the 2nd balancing circuit network maximizing a howling margin in a four-wire line loop.

CONSTITUTION: An inverting input terminal (-) of a balancing input amplifier A1 and an output terminal of an amplifier A2 are connected via the 1st balancing circuit network ZN determining the input impedance viewed from the two- wire line I . Further, a non-inverting input terminal (+) of the balancing input amplifier A1 and an output of the amplifier A2 are connected via the 2nd balancing circuit network Z12 determining the howling margin in a four-wire line IIloop. Further, the 1st balancing circuit network ZN is set optionally so as to provide a specific impedance. Further, Z1, Z2 of the 2nd balancing are set so as to maximize the howling margin taking line impedances ZL, ZN connected to the two-wire line into account.