Speed reducer

A speed reducer comprising:

• a ring-shaped external gear including a plurality of outer teeth and having a first through hole and a plurality of second through holes, the outer teeth being arranged on an outer circumferential surface of the external gear along circumferential directions, the first through hole penetrating the external gear in axial directions, the second through holes being arranged in the circumferential directions to surround the first through hole;
• an input shaft penetrating the first through hole and rotatable about a rotation axis;
• a first bearing disposed between the external gear and the input shaft and holding the input shaft such that the input shaft is rotatable relative to the external gear in the circumferential directions;
• a plurality of inner pins penetrating the second through holes in the axial directions;
• an inner pin holder holding both ends of the inner pins and surrounding an outer circumferential surface of the input shaft;
• a second bearing surrounding the outer circumferential surface of the external gear;
• and
• internal tooth pins meshing with the outer teeth, wherein
• the second bearing includes
• an outer ring fixed to the inner pin holder,
• an inner ring disposed inward of the outer ring in radial directions and constituting an output shaft that rotates at a speed lower than the input shaft, and
• a roller that contacts an inner circumferential surface of the outer ring and an outer circumferential surface of the inner ring, and
• the inner ring includes a pin holding portion having a ring shape surrounding the outer circumferential surface of the external gear, the pin holding portion holding the internal tooth pin and extending along an inner circumferential surface of the inner ring.