A swing speed reducer comprising: a main body; an input shaft that is rotatably held in the main body; a precession body that precesses; a swinging body that engages with the precession body and is swung by the precession; and an output shaft that is rotated by the swing of the swinging body; wherein a precession annular groove for precession of the precession body and a swinging annular groove for swinging the swinging body are provided on the surface of a spherical portion.