The present disclosure provides a rotational angle detecting device. An IC substrate has a flat surface extending along a rotational axis of a throttle valve. Yokes forms, together with the magnets, a closed magnetic circuit. A first Hall element outputs a first signal according to the magnetic flux density in a first direction along the flat surface. A second Hall element outputs a second signal according to the magnetic flux density in a second direction intersecting the flat surface. The first and second Hall elements are positioned within a region that is surrounded by the magnets and the yoke and that is between an edge surface and an edge surface.