A first shaft end of a hollow shaft of a hollow strain wave gearing is supported by a first support bearing with a collar interposed therebetween which is mounted to the outer-circumferential surface of the first shaft end. A second shaft end on the other end of the hollow shaft is supported by a second support bearing. A wave plug of a wave generator is integrally formed with the hollow shaft, and a wave bearing is mounted on the elliptical outer-circumferential surface thereof from the first-shaft-end side. It is possible to use a first support bearing having an inner diameter larger than the outer diameter of the first shaft end as the first support bearing for supporting the first shaft end.