One end of a rotational shaft of a supporting rotational body attached with a rotational member is rotatably supported. The rotational member is sandwiched by a supporting frame that rotatably supports supporting rotational bodies spanned with an annular belt and fixing members fixed to the supporting frame. Thus, the one end of the rotational shaft is supported by the supporting frame and the fixing members at, at least two points. A mark detector is fixed to the fixing members, and the fixing members are fixed to the supporting frame at, at least two points. Accordingly, erroneous detection of a rotational angular speed or a rotational angular displacement of a supporting rotational body due to vibration of the supporting rotational body can be suppressed.