PLANETARY GEAR MECHANISM

A planetary gear mechanism (1) of the invention includes a housing H, an input shaft (2) that has an eccentric portion (22) whose center lies on an eccentric axis, and is rotatable about the axis of input and output shafts relative to the housing (H), an oscillating face plate (3) that eccentrically oscillates relative to the input shaft (2) via the eccentric portion (22), and rotates about the eccentric axis, and an output shaft (7) that is provided with a second internally toothed gear that internally contacts and meshes with a second externally toothed gear formed on the oscillating face plate (3), and is rotatable about the axis of the input and output shafts relative to the housing (H). The housing (H) is formed with a first internally toothed gear that internally contacts and meshes with a first externally toothed, gear formed on the oscillating face plate (3). As the input shaft (2) rotates, a circumferential position at which each externally toothed gear meshes with the corresponding internally toothed gear changes, and rotation of the input shaft (2) is transmitted at a reduced speed to the oscillating face plate (3), of which rotation is delivered at a further reduced speed to the output shaft (7).