PURPOSE: To provide a rotation booster which is simple in structure but high in power amplifying factor.

CONSTITUTION: A device is equipped with epicyclic mechanisms 15 and 15' in plural stages to which a rotation booster is mutually and drivingly connected. In the device, the epicyclic mechanism in each stage allows each large diameter eccentric boosting gear 22 and 22' to be engaged with each small diameter drive gear 21 and 21' driven by each input shaft 15b and 15c, and also allows each eccentric boosting gear to be connected to each epicyclic gear 28 and 28' which is small in diameter than the eccentric boosting gear while being rotated integrally therewith. By this constitution it is so designed that the torque of the drive gears 21 and 21' is transmitted to the epicyclic gears 28 and 28' while being multiplied in torque, so that the amplified torque is thereby outputted out of carriers 31 and 31' connected to the epicyclic gears.