A hollow strain-wave gear device (1) has a wave generator (5), a wave generator plug (5a) of which is integrally formed in a portion of a hollow rotating shaft (6). A plug reinforcement part (5d), which is adjacent to the wave generator plug (5a), has a thickness larger than that of the wave generator plug (5a), and is integrally formed in the hollow rotating shaft (6). The wave generator plug (5a) is reinforced by the highly rigid plug reinforcement part (5d) formed adjacently to the wave generator plug (5a). It is thus possible to prevent or suppress a decline in the rigidity of the wave generator plug (5a) when the hollow rotating shaft has a thin thickness in order to reduce the weight of the hollow rotating shaft (6) and increase the diameter of the hollow portion thereof. Consequently, it is possible to suppress a decline in ratcheting torque.