PURPOSE: To improve measurement accuracy by changing over the polarity of a current to be allowed to flow in a coil for every specified cycle, smoothing the absolute value obtained from the output of a Hall element and outputting.

CONSTITUTION: A magnetic field generated proportionally to the current i1 flowing in the coils 3, 4 is detected by the Hall element 7 adjacent to an iron core 5 wound on the coils 3, 4. At this time, the coils 3, 4 are wound so that the direction of magnetic field for the iron core 5 is symmetrically altered, and switches 1, 2 are altenately connected to the coils 3, 4 for every specified cycle by a rectangular wave generator 8. Then, the output of element 7 is amplified by a differential amplifier 9, and frequency components in the output of absolute value converted by an absolute value circuit 10 are eliminated by an LPF 11, then that value is smoothed and outputted. Thus, the accuracy for measurement can be improved.