Internal meshing planetary gear device and joint device for robot

The invention provides an inner joint planetary gear device and a joint device for a robot. The reliability of the inner joint planetary gear device is not easy to decrease. An internally engaged planetary gear device (1A) includes an internally toothed gear (2), a planetary gear (3), and a bearing member (6). The bearing member (6) has an inner ring (61) and an outer ring (62). A ring-engaged planetary gear device (1A) rotates a planetary gear (3) relative to an internally toothed gear (2) by swinging the planetary gear (3), and rotates an outer ring (62) relative to an inner ring (61) about a rotation axis (Ax1). A gap is provided between the inner ring (61) and the outer ring (62) in the axial direction along the rotating shaft (Ax1), and a preload is applied between the inner ring (61) and the outer ring (62) by the elastic part (8) so as to fill the gap.