Harmonic gear device and robot joint device

Provided are a harmonic gear device and a joint device for a robot that are not susceptible to a decrease in reliability. A harmonic gear device (1) is provided with a rigid internal gear (2), a flexible external gear (3), and a wave generator (4) having a non-circular cam (41) that is driven to rotate about a rotation axis (Ax1), and a bearing (42) that is attached to the outside of the cam (41), the wave generator (4) deforming the flexible external gear (3) in accordance with the rotation of the cam (41), causing a part of external teeth (31) to mesh with a part of internal teeth (21), and causing a part of external teeth (31) to mesh with a part of internal teeth (21). The flexible external gear (3) rotates relative to the rigid internal gear (2) according to the tooth number difference between the flexible external gear (3) and the rigid internal gear (2), and a chemically converted coating (C1) is arranged on a target surface (S1) formed by at least one of the outer peripheral surface (424) of the wave generator (4) and the inner peripheral surface (301) of the flexible external gear (3).