Abstract: An eccentric oscillation gear device includes a casing, a first member supported by the casing via a first bearing, a second member supported by the casing via a second bearing, and a fastening portion fastening the first and second members in an axial direction of the casing. The fastening portion includes an internally threaded portion formed in the first member, and a fastener having an externally threaded portion. The fastener has a Rockwell hardness (HRC) of 44 or higher, and the internally threaded portion has a lower hardness than the fastener.

Abstract: An eccentric oscillation gear device according to the disclosure includes: a casing; a first member supported by the casing via a first bearing; a second member supported by the casing via a second bearing; and a fastening portion fastening the first member and the second member to each other in an axial direction of the casing. The fastening portion includes an externally threaded portion and an internally threaded portion. At least one selected from the group consisting of feet of threads of the externally threaded portion and roots of threads of the internally threaded portion has elasticity portions for facilitating elastic deformation of the threads associated with the elasticity portions when the externally threaded portion and the internally threaded portion are tightened to each other.