A first opposing surface of a first yoke has a constant radius of curvature and has a center of curvature that lies on a center of rotation. At least two rotation direction end portions of a second opposing surface of the second yoke are formed such that toward either end in the rotation direction, the two end portions are increasingly spaced away from a reference curved surface in a direction away from the first opposing surface. The first and second opposing surfaces face each other to define a gap thereinbetween. Accordingly, the gap between the yokes is wider at its end portions than at its center, and increases in magnetic flux density flowing through a Hall element is curtailed at the end portions of the gap. Further, it is not necessary for the first yoke to protrude outward when widening the end portions of the gap.