An oscillating inner gearing planetary gear system is provided which does not need to occupy a large space even in a state where a driving source is joined. In particular, the axial length thereof can be shortened, and a large diameter hollow shaft therethrough can be formed. The oscillating inner gearing planetary gear system (100) is configured in such a manner that rotation of an input shaft (104) is reduced by internal oscillating bodies (116A,116B) oscillatingly rotating relative to an external gear (118), and the reduced output is delivered by the external gear (118) also serving as an output shaft. A middle shaft (108) is located parallel to the external gear (118) at a position more radially outward than the internal oscillating bodies (116A,116B). An orthogonal gearset (106) connects the middle shaft (108) and the input shaft (104) at a right angle. Power from the input shaft (104) is inputted from a direction radially outward of the internal oscillating bodies (116A,116B) through the middle shaft (108).