Provided is a reduction gear with which allowable output can be further increased and a reduction in size can be achieved. A reduction gear (1) has a case (2), a speed reduction mechanism (3) accommodated within the case (2), a shaft portion (51) to which output of the speed reduction mechanism (3) is transmitted, and a pinion (52) coupled to the shaft portion (51). The speed reduction mechanism (3) has an external tooth gear (15) to which output of an electric motor (100) is transmitted, and a carrier (17) holding the external tooth gear (15). The pinion (52) and the shaft portion (51) are integrally formed, and the shaft portion (51) and the carrier (17) are spline-coupled to each other. A pair of main bearings (32, 33) are held by the case (2), and rotatably support the carrier (17).