To absorb misalignment of eccentric bodies, bearings, and external tooth gears, enhance the capacity of high loading, and miniaturize a reduction gear by forming the bearing interposed between the eccentric bodies and the external tooth gears of a single curvature radius and forming the outer circumferences of the eccentric bodies and the internal circumferences of the external tooth gears along the curvature of the bearings.

Bearings 24a, 24b are so constituted that the shapes of the bearing outer circumferences is formed of a single continuous curvature. Eccentric bodies 23a, 23b have their outer circumferences formed along the shapes of the outer circumferences of the bearings 24a, 24b. When misalignment is 0, it is natural, however, when the misalignment becomes large because of high load, this constitution allows the generation of misalignment in the eccentric bodies 23a, 23b, the bearings 24a, 24b, and the external tooth gears 25a, 25b so as to hold a constant loading state.