A first decelerator is placed in an arm base such that a lower end of a decelerator shaft is bared in the arm base. A first arm has a hermetic space which becomes equal in pressure to a hermetic space of the arm base when an upper end of the hollow decelerator shaft is inserted thereinto, and is secured to a first decelerator output shaft. A second decelerator is placed on a distal end of the first arm, and has an input shaft connected to the decelerator shaft. A second arm is secured to an output shaft of the second decelerator, and has no hermetic space formed therein. A link mechanism follows the first and second arms.