FIG. 4A is a cross-sectional view including the central axis of the core 106, and FIG. 4B is a cross-sectional view taken along a line X-X??? in the FIG. 4A. As in FIG. 1A, in FIG. 4A and other drawings showing the similar configuration, for clarification of the drawing, the vicinity of the central axis of the core 106 and the lines of magnetic force (dotted lines) are shown in cross section, and