Abstract: A speed reducer includes a sun gear, planetary gears, a fixed part, an output part, a first bearing, and a second bearing. The sun gear rotates relative to the fixed part at a first rotation speed. The planetary gears are disposed around the sun gear to engage with the sun gear. The output part includes an annular internal gear engaging with the planetary gears. The output part rotates relative to the fixed part at a second rotation speed lower than the first rotation speed. The first bearing is interposed between the fixed part and the output part at a position closer to an input side with respect to the planetary gears. The second bearing is interposed between the fixed part and the output part at a position closer to an output side with respect to the planetary gears. The second bearing is smaller in radial size than the first bearing.