To drive a motor at the rotational speed of high efficiency, and to reduce vibration and impact between a driving side and a driven side.

The rotational force transmission device comprises a rotational transmission mechanism provided with a first rotating shaft 123 and a second rotating shaft 124, which are parallel to each other, a first housing part 111 which incorporates the rotation transmission mechanism and supports the first rotating shaft, and a second housing part 112 which rotates relative to the first housing part with the first or second rotating shaft as a rotation axis center and supports the second rotating shaft. The first housing part and the second housing part are provided with a rotational force suppressing mechanism 13 which holds the first and second housing parts in a relative reference position when there is no relative rotational force between the first and second housing parts, while controls the position of the first and second housing parts against the relative reference position when the first and second housing parts rotate relatively.