PURPOSE: To accurately measure a current that exceeds detection ranges of various kinds of sensors by detecting a commercial frequency, a provisional amplitude and an error between a value coincident with a sampling time of a sinusoidal wave because of a phase advance angle obtained by Fourier transform.

CONSTITUTION: A secondary side current is taken via a current transformer CT to be A/D converted and digital data is sampled at prescribed time intervals, then the data is Fourier transformed by means of an FFT. It is judged whether the obtained amplitude 1m exceeds a predetermined reference value 1d. When it is equal to or greater than the reference value 1d, a sinusoidal wave is formed. An obtained phase advance angle θ is used for the forming of the sinusoidal wave and the reference value 1d at the beginning is used for a provisional amplitude (Is). Next, coded value is calculated at time intervals (t) coincident with the sampling time of the sinusoidal wave. A total error of instant value data and sampling data of the sinusoidal wave is calculated. Then, the amplitude (Is) is increased to be compared to the total error before the increasing. When the error is increased, the calculation is terminated.