Provided is a speed reducer including: a first reduction unit disposed at an input stage of a multi-stage reduction unit that decelerates a rotational driving force of an electric motor from an input stage side toward an output stage side and transmits the decelerated rotational driving force to a rotational drive unit; a first ring gear meshed with first planetary gears provided in the first reduction unit; and a third reduction unit disposed at an output stage of the multi-stage reduction unit and having a first case. The first ring gear is provided in the first case of the third reduction unit.