To provide a speed reducer capable of commonalizing core components in regard to speed reducers with different load conditions or the like acting between rotated members.

A speed reduction unit 50 includes a cylindrical member 56 formed with internal teeth 48 on an inner circumference, and carriers 16 (16X, 16Y) having external gears 42X, 42Y arranged coaxially with the cylindrical member 56, rotatably supported by the cylindrical member 56 via first bearings 10X, 10Y, and meshed with the internal teeth 48. A support unit 64 includes first support members 4 (4X, 4Y) unrotatably mounted on an outer circumference of the cylindrical member 56, and second support members 2 (2X, 2Y) unrotatably mounted to the carriers 16 and rotatably supported by the first support members 4 via second bearings 68X, 68Y. The speed reduction unit 50 and the support unit 56 can be separated.