PROBLEM TO BE SOLVED: To provide a reliable speed reduction gear capable of preventing outbreaks of a lock phenomenon in a speed reduction mechanism section when an excessive torque is applied to an eccentric disk.SOLUTION: An internal gear 3 is supported by a housing 1 which covers shaft end parts of both of an input shaft 7 and an output shaft 12. The eccentric disk 9, which can rotate in the internal gear 3, is provided in the shaft end part of the input shaft 7, and rolling bodies 19, which can roll along the outer radial surface of an outer ring of a bearing 11, are housed in a plurality of pockets 18 of a cage 14 disposed between the internal gear 3 and the bearing 11, which is pressed into the outer radial surface of the eccentric disk 9, in the shaft end part of the output shaft 12. The rotation of the input shaft 7 is reduced by the number of the internal teeth 4, and is transmitted to the output shaft 12. The eccentric disk 9 is configured to freely rotate relative to the input shaft 7, and a torque limiter mechanism 20, which cuts the transmission of the torque from the input shaft 7 to the eccentric disk 9 when the rotating torque being applied to the eccentric disk 9 exceeds a set torque, is provided between the eccentric disk 9 and the input shaft 7 so as to prevent outbreaks of a lock phenomenon.