In a placement face of which a general face is configured with a circular cone shape, restriction faces are formed. When a sensor ball rolls along an axial direction of rotating shafts, the sensor ball accordingly contacts one of the restriction faces or another of the restriction faces. From this state, in order for the sensor ball to roll further along the axial direction of the rotating shafts, the sensor ball has to rise up the one of the restriction faces or the other of the restriction faces that respectively have a larger angle of incline than the general face. By contrast, in order for the sensor ball to roll in directions orthogonal to the axial direction of the rotating shafts as seen in plan view, the sensor ball only has to roll over the general face.