PURPOSE: To attain the balancing of unbalanced attenuation to ground by subtracting one output of a potential detection circuit with other output via a gain variable circuit and a frequency characteristic variable circuit and feeding back the output difference value to the gain variable circuit and the frequency variable circuit.

CONSTITUTION: If the impedance of a feeding circuit 11 to an in-phase current is unbalanced, the outputs of potential detection circuits 12, 13 differ and a signal is detected to the output point P of the subtraction circuit. Thus, a control circuit 22 controlling each characteristic of gain/frequency is operated so as to control circuits 20, 21. Then the potential at the point P is checked and when it is not zero, the circuit 22 is operated again. This is repeated and the potential at the point P is converged within a prescribed range. This operation is processed automatically digitally and performed by a digital signal processing circuit 17 including HPFs 18, 19, the gain variable circuit 20, the frequency characteristic variable circuit 21 and the control circuit 22.