PURPOSE: To absorb an impact torque and reduce the generation of a torsional vibration.

CONSTITUTION: In a flexible gear joint in which a pair of first internal gears 3a provided co-axially with a sleeve 3 are meshed with a first external gear 9a provided on a pair of pinions 9, a first tooth 9b of a second internal gear is formed on the inner periphery of the pinion 9, a second tooth 8a opposed to the first tooth 9b at a specified interval is formed on the outer periphery of a boss 8 fixed to a rotating shaft 1, and an elastic body such as nitrile rubber is filled between the first tooth 9b and the second tooth 8a. Thus an impact torque and a torsional vibration from the rotating shaft l can be absorbed.