Eccentric differential system speed reducer

(57) while securing the hollow space penetrating the center portion of the Abstract OBJECTIVE eccentric differential type speed reducer, to improve the degree of freedom in setting the reduction ratio. [Configuration] ring gear to mesh with each of the plurality of crank shaft 34 to the spur gear 38 fixed respectively 48 While disposed, its input gear 52 to mesh with the ring gear 48 provided eccentrically from a center line L, the holding member 14 and the first through the central portion of the eccentric gear 22 drive shaft 60 and the second the drive shaft 62 It was to be disposed, and the like.