The present invention relates to a ring gear and a gear reduction device. The ring gear includes an annular body with a plurality of mounting portions, and a plurality of teeth arranged on an inner circumferential surface of the annular body. The annular body and the teeth are integrally formed from powder metal, and the mounting portions and the teeth are at least partly overlapped in an axial direction of the annular body. The mounting portions are integrally formed with the annular body and the teeth, which results in concentricity between the mounting portion and the teeth being improved and cost of the ring gear being reduced. Furthermore, the mounting portions are overlapped with the teeth, which reduces the axial size of the ring gear.