PURPOSE: To extract a signal superimposed on a distribution line or the like based on a voltage induced in a coil, by providing the coil contactlessly with a core wire of the distribution line or the like at a location subject to the effect of a magnetic ield produced around the distribution line.

CONSTITUTION: The magnetic field H at a distance (r) to the coil 6 is expressed as H=I/2πr [AT/m]. The total magnetic flux is expressed as Equation 1, where B=μH [wb/m2] is magnetic flux density and (a) is radius of the coil 6 in [m]. Further, a voltage E induced in the coil 6 is expressed in Equation 2, where N is the number of windings of the coil 6. The induced voltage E is in- creased with the coil 6 wound on a ferromagnetic substance 7. Since the relation of E0/E=f0/f is established, where f[Hz] is commercial frequency, f0[Hz] is signal frequency, and E[V], E0[V] are output voltages at respective magnetic field, a signal superimosed is detected on the basis of the induced voltage of the coil 6.