Mutual interference between rotation and turning drive forces is prevented by a simple structure. A transmission shaft and a turning body are supported at a housing so as to be relatively coaxially rotatable. An input shaft is connected to the transmission shaft, and a first sun gear is fixed thereon. First planetary gears meshing with the first sun gear are supported at a rotating body, which is connected to an output shaft. A second sun gear is provided on a hollow-cylindrical shaft fixed to the housing. Second planetary gears supported at the turning body mesh with the second sun gear. A first and second inner gears mesh with the first and second planetary gears respectively. They are corotatably connected by a connection ring. A rotating shaft is perpendicularly connected to the output shaft through bevel gears. A casing supporting the output and rotating shafts and is joined to the turning body.