Internal meshing planetary gear device and joint device for robot

The invention provides an internally engaged planetary gear device and a joint device for a robot, which are easy to suppress an increase in backlash. The internally engaged planetary gear device (1A) includes an internally toothed gear (2) and a planetary gear (3). The internally toothed gear (2) has an annular gear body (22) and a plurality of pins (23). The plurality of pins (23) are held in a plurality of inner circumferential grooves formed in the inner circumferential surface of the gear body (22) in a rotatable state, and constitute internal teeth (21). The planetary gear (3) has outer teeth (31) that partially mesh with the inner teeth (21). The internally engaged planetary gear device (1A) causes a planetary gear (3) to rotate relative to an internally toothed gear (2) by swinging the planetary gear (3). The gear body (22) has an elastically deformable portion (24) that is elastically deformable at least in a location where the plurality of inner peripheral grooves (223) are formed.