PURPOSE: To remove or reduce an angle backlash in an internally meshing planetary gear mechanism in which a plurality of eccentric bodies penetrate an external gear by defining an input shaft as a motor output rotational shaft, and by having the input shaft penetrated between a support block and a carrier with a gap formed therebetween.

CONSTITUTION: An internal gear 10 has the internal teeth of an external pin 11. External gears 81, 82 have a curved inner circmrerential surface 12, and a support block 13 is inserted therein. A carrier 14 is fixed to the end part of the support block 13 by a pin 15 and a bolt 16. An input shaft 1 penetrates a part between the support block 13 and the inserting port in the vicinity of the center of the carrier 14, with a gap formed, and the pinion 2 on the end part and a transmission gear 3 on the end part of an eccentric body shaft 4 are meshed with one another. Since angle backlash occurs in gear transmission, a transmission gear 31 is displaced by the same degree of the angle backlash in the direction of normal rotation, and a transmission gear 32 is displaced by the same degree in the direction of reverse rotation. Since there are a small number of connection mechanism between an output rotational shaft and the rotation shaft of the pinion, the angle backlash can be reduced to the same degree.