To detect the presence or absence of a magnetic object accurately winding a coil around a core at a plurality of points, canceling a high-frequency magnetic flux that changes by the presence or absence of the magnetic substance each other, and reducing the leakage of a high-frequency magnetic flux toward the outside of a detection region.

A core 3 is formed in nearly E shape where extension pieces 3b, 3c, and 3d are extended in parallel one another in the same direction from both edge parts and the center part of a base piece 3a. The core 3 is wound around a coil 4 along with a coil bobbin 2 where the coil is placed. Then, both intermediate parts 3e and 3f of the core 3 become a part 3g to be wound by the coil 4 and the coil 4 is wound in opposite direction. Therefore, by canceling out a high-frequency magnetic flux being generated from each winding part of the coil 4, the leakage of the high-frequency magnetic flux toward the outside of the detection region of a through hole can be reduced, thus allowing a magnetic substance to be located at a detection region and achieving an accurate detection without reducing excessively as compared with an original reduction when the eddy current loss of the high-frequency magnetic flux through the detection region is generated and an oscillation amplitude is reduced.