PURPOSE: To mitigate machining accuracy and to suppress backlash by arranging a pair of small gears having identical compositions in parallel with a large gear and arranging a pair of helical gears having adjustable gearing position between the rotary shafts of the small gears.

CONSTITUTION: A pair of small drive gears 11, 12 having identical compositions are engaged with a large ring gear 1, and the first drive gear 11 is secured together with a helical gear 15 to a first drive shaft 13 which is driven by a hydraulic motor 4 through a reduction gear 5. The second drive gear 21 is interlocked with a helical gear 16 which is spline coupled with an intermediate shaft 31 through spar gears 25, 26. The helical gear is positioned through a bellevelle spring 35 and engaged with the helical gear 15. By such arrangement, machining accuracy can be mitigated and backlash can be suppressed.