A magnetic core surrounds a coil and has a plurality of first core sections arrayed along the widthwise direction of a recording medium orthogonally to the conveyance direction of the recording medium, and a second core section disposed at both ends in the widthwise direction within a hollow section of the coil. The second core section is formed so that the cross-sectional area thereof in the conveyance direction of the recording medium grows progressively larger from the center of the widthwise direction towards the end thereof.