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 . <EOS>