Axial positions of two driven external gears 66 making up each of a plurality of paired-gear groups 67 with respect to an input shaft 62 are made identical, and a different paired-gear group 67 is arranged by being offset in the axial direction of the input shaft 62. Therefore, only two driven external gears 66 making up the paired-gear group 67 are present at an identical axial position of the input shaft 62. Consequently, even if the diameters of these driven external gears 66 are made large up to the very limit, these driven external gears 66 do not interfere with each other. Accordingly, it is possible to readily enlarge the reduction ratio based on the driving and driven external gears 65 and 66.