a magnetic sensor including an annular iron core , an annular hollow bobbin for a primary winding which incorporates therein the annular iron core , the bobbin having a plurality of protruded portions which are arranged along a circumference of the annular bobbin with an equal distance , each of the protruded portions being extended in a direction parallel to a center axis of the annular iron core , the primary winding being wound on the annular bobbin at respective portions between adjacent protruded portions with an equal number of turns , a bobbin for a secondary winding having a positioning means for the primary winding in association with the protruded portions of the bobbin for the primary winding when the bobbins for the primary and secondary windings are assembled , the secondary winding being wound on the bobbin to form more than two pairs of winding groups .