PURPOSE: To eliminate the mutual interference between magnets, by magnetizing the N and S poles at one end of the element alternately through the revolution of a permanent magnet to the induction element, and inducing the pulse electromotive force to the detection coil placed near the inducing element.

CONSTITUTION: The inducing element 1 consists of the ferromagnetic substance of comparatively soft and hard magnetism anisotropy, and the permanent magnet 3 is revolved to the element 1. Further, the N and S poles are alternately magnetized at one end of the element 1 and the element 1 is alternately magnetized to positive and negative direction. Thus, the pulsive electromotive force is induced in the detection coil 2 placed near the element 1 to eleiminate the mutual interference between the magnets.