First and second divisional cores each including right and left leg portions and a yoke interconnecting those together are formed by molding respective yoke-side core members in a resin. Cylindrical core mounting portions extending from the outer circumference of the surface of the yoke-side core member are formed integrally with the respective right and left leg portions of the first divisional core. I-shaped leg-portion-side core members and spacers are attached in the cylindrical core mounting portion formed in each of the right and left leg portions. The surface of the yoke-side core member molded in the resin and the surface of the leg-portion-side core member are disposed so as to have a spacer therebetween. The two divisional cores are joined together by butting respective leg portions of the two divisional cores with each other to form an annular mold core, and a coil is wound around the mold core.