A speed reducer according to the invention includes: a main speed reduction unit decelerating rotation from an input side and transmitting decelerated rotation to an output side; a fixed block coupled to the main speed reduction unit and rotatably supporting an input gear coupled to a rotary drive source; an output gear transmitting rotation of the input gear to an input side of the main speed reduction unit; and an intermediate gear meshing with the input gear and the output gear and rotatably supported by the fixed block.