A rotating body, which rotates about an axis, includes a recessed reference mark and at least one recessed dummy mark. The reference mark is provided on the surface of the rotating body, serves as a reference for detecting the rotational phase of the rotating body, and can be detected with an electromagnetic wave. The dummy mark is provided on the surface of the rotating body, and is located at a position separated from the reference mark by an angle greater than 90 degrees about the axis.