PURPOSE: To make only the speed of rotation of a main rotor variable without changing engine speed and causing shock in the power transmission of a compound type helicopter by allowing a ring gear to freely rotate, and providing a brake means which stops the ring gear from rotating about its axis.

CONSTITUTION: A ring gear 6 is allowed to freely rotate as its inner teeth mesh with an epicyclic gear 5. A brake 7 is provided which stops and releases rotation of the ring gear 6. The ring gear 6 is driven by a plurality of speed change pinions 12 each of which meshes with the inner teeth of the ring gear 6 and by a plurality of speed change gears 14 each of which meshes with a free-wheel-equipped speed change sun gear 15 mounted on the rotary shaft 13 of the speed change pinions 12. When the brake 7 is actuated the ring 6 is not rotated but actuated in the same way as a normal epicyclic reduction gear. When the brake 7 is released, the ring gear 6 rotates in the opposite direction to rotation of a main rotor drive shaft 18 because of reaction and the speed of a rotor is lowered by an amount of the rotation.