The present invention provides a ring gear and a gear reduction device. The ring gear includes a one-piece annular body made from powder metallurgy material, and a first ring gear part and a second ring gear part formed on a surface of the annular body. The first ring gear part and the second ring gear part are arranged along an axial direction of the annular body. The ring gear has a density that is less than 92% of its theoretical density and in the range of 6.0 to 8.0 grams per cubic centimeter. The present invention further provides a mold for manufacturing the ring gear. The ring gear of the present invention includes two ring gears integrally formed at once, and the two ring gears have improved concentricity.