PURPOSE: To eliminate a trouble by differential planetary action by driving with an output shaft a gear wheel, arranging in its peripheral edge fixed position a pinion and mounting a planet gear to a pinion shaft and a sun gear and an internal gear to the internal periphery and periphery of the planet gear.

CONSTITUTION: Rotation of a main engine 1 rotates a gear wheel 3 rotating by its rotation a gear shaft 14 rotating a planet gear 5. Rotation of the planet gear 5 rotates a sun gear 8 in the same direction on the one hand and an internal gear 6 in the reverse direction on the other hand. Two or more pinions 4, meshed with the periphery of the gear wheel 3, and the planet gear 5, associated with the gear shaft 14, are not rotated about the sun gear but rotated on the own axis because a position of the gear shaft 14 is restricted. Accordingly, the sun gear 8, connected to a rear propeller 12, is rotated in the same direction to the gear wheel 3, and the internal gear 6, connected to a front propeller 11, is rotated in the reverse direction. Accordingly, a trouble in the point of cost is eliminated with no differential planetary action.