A gear device comprising: a first planetary gear train including a first sun gear, a first planetary gear set surrounding the first sun gear, and a first carrier that is driven by revolution of the first planetary gear set about the first sun gear; a second planetary gear train including a second sun gear, a second planetary gear set surrounding the second sun gear, and a second carrier that is driven by revolution of the second planetary gear set about the second sun gear; and a ring gear comprising: a first ring gear part comprising a first annular body and a plurality of first teeth formed on a surface of the first annular body; and a second ring gear part comprising a second annular body and a plurality of second teeth formed on a surface of the second annular body; and a flange mounted to an end of the ring gear, wherein the flange comprises a flange body abutting the end of the ring gear, and a connecting portion extending from the flange body and being inserted into inside of the ring gear; characterized in that the first ring gear part and the second ring gear part are integrally formed from powder metal, the ring gear having a mass of 6.0 to 8.0 grams per cubic centimeter; and the first planetary gear train and the second planetary gear train are disposed within the ring gear, each planetary gear of the first planetary gear set meshing with both of the first sun gear and the first teeth of the ring gear, and each planetary gear of the second planetary gear set meshing with both of the second sun gear and the second teeth of the ring gear; and wherein the connecting portion has a round circumferential surface configured to contact with the tips of the teeth of the ring gear. a first planetary gear train including a first sun gear, a first planetary gear set surrounding the first sun gear, and a first carrier that is driven by revolution of the first planetary gear set about the first sun gear; a second planetary gear train including a second sun gear, a second planetary gear set surrounding the second sun gear, and a second carrier that is driven by revolution of the second planetary gear set about the second sun gear; and a ring gear comprising: a first ring gear part comprising a first annular body and a plurality of first teeth formed on a surface of the first annular body; and a second ring gear part comprising a second annular body and a plurality of second teeth formed on a surface of the second annular body; and a first ring gear part comprising a first annular body and a plurality of first teeth formed on a surface of the first annular body; and a second ring gear part comprising a second annular body and a plurality of second teeth formed on a surface of the second annular body; and a flange mounted to an end of the ring gear, wherein the flange comprises a flange body abutting the end of the ring gear, and a connecting portion extending from the flange body and being inserted into inside of the ring gear; characterized in that the first ring gear part and the second ring gear part are integrally formed from powder metal, the ring gear having a mass of 6.0 to 8.0 grams per cubic centimeter; and the first planetary gear train and the second planetary gear train are disposed within the ring gear, each planetary gear of the first planetary gear set meshing with both of the first sun gear and the first teeth of the ring gear, and each planetary gear of the second planetary gear set meshing with both of the second sun gear and the second teeth of the ring gear; and wherein the connecting portion has a round circumferential surface configured to contact with the tips of the teeth of the ring gear.