Speed reducer

A speed reducer comprising:

• a ring-shaped external gear including a plurality of outer teeth and having a plurality of through holes, the outer teeth being arranged on an outer circumferential surface of the external gear in circumferential directions, the through holes penetrating the external gear in axial directions and arranged in the circumferential directions;
• an input shaft penetrating space surrounded by an inner circumferential surface of the external gear and extending in the axial directions;
• a 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 ring-shaped internal gear including a plurality of inner teeth and surrounding the outer circumferential surface of the external gear, the inner teeth being arranged on an inner circumferential surface of the internal gear and meshing with the outer teeth;
• a plurality of inner pins penetrating the through holes in the axial directions;
• and
• an inner pin holder holding both ends of each of the inner pins and having space, the input shaft penetrating the space, wherein
• the inner pin holder includes
• a first holder portion including a ring-shaped holding portion holding a first end of each of the inner pins,
• a second holder portion including a ring-shaped holding portion holding a second end of each of the inner pins, the second end being opposite to the first end, and
• pillars connecting the first holder portion and the second holder portion to each other, arranged at intervals in the circumferential directions, and penetrating the through holes,
• the speed reducer further comprises fixing members that fix end surfaces of the pillars to the first holder portion or the second holder portion and are inserted in the pillars from the end surfaces of the pillars,
• two inner pins of the inner pins and one pillar of the pillars are disposed in a same through hole of the through holes, and
• the two inner pins sandwich the one pillar in the circumferential direction.