Speed reducer and actuator

A speed reducer for converting rotational motion about a rotation axis extending between an input side and an output side, at a first rotation speed into rotational motion at a second rotation speed being lower than the first rotation speed, the speed reducer comprising:

• a fixed part;
• a sun gear that rotates relative to the fixed part about the rotation axis at the first rotation speed;
• a plurality of planetary gears that are disposed around the sun gear to engage with the sun gear;
• an output part that includes an internal gear having an annular shape and engaging with the plurality of planetary gears and rotates relative to the fixed part at the second rotation speed;
• a first bearing that is interposed between the fixed part and the output part at a position closer to the input side with respect to the plurality of planetary gears;
• and
• a second bearing that is interposed between the fixed part and the output part at a position closer to the output side with respect to the plurality of planetary gears, wherein
• a radial size of the second bearing is smaller than a radial size of the first bearing.