A rotational mechanism includes a frame, a plurality of rotational units, an inner ring rotatably supported on the frame via the plurality of rotational units, and a rotation driving unit configured to generate power for rotating the inner ring. The inner ring is provided with a protrusion portion on an outer periphery. Each of the plurality of rotational units includes two bearings and a first pressing unit. The first pressing unit is configured to apply pressing forces to the two bearings in opposite directions in a rotational axis direction of the inner race. The protrusion portion is held between the two bearings due to the pressing forces.