A cup-type hollow strain wave gearing of a hollow actuator has a seal part assembled inside a cup-shaped externally toothed gear. The seal part seals between a boss of the externally toothed gear and a plug of the wave generator so as to prevent lubricant from leaking from the side of the externally toothed gear into a hollow part. Since the seal part employs an oil seal and an O-ring, it is able to be constituted inexpensively and to surely prevent leakage of low-viscosity lubricant.