PURPOSE: To detect the exciting current of a transformer accurately by determining the phase angle of a detected exciting current with respect to flux and then correcting the detected primary current and/or secondary current so that the phase angle is nullified.

CONSTITUTION: Primary current I1 and secondary current I2 are detected by current detectors CT1, CT2. The detected value I2 of the secondary current is multiplied by a2/a1 through a proportional circuit K and expressed in terms of a primary current I2' which is then corrected through a coefficient corrector H and fed to an adder/subtractor A. On the other hand, the detected value I1 of the primary current is fed, as it is, to the adder/subtractor A which produces an exciting current I0. A phase angle operating unit CAL operates the phase shift ψ of the detected exciting current I0 from flux ψ0 of a transformer TR. The coefficient corrector H regulates the correction coefficient K depending on the phase angle ψ and increases K if ψ is positive otherwise decreases K.