In this hollow strain wave gearing unit, a shaft end part of a hollow rotary shaft of a wave generator is located inside a cup-shaped flexible external gear. The shaft end part is supported by a bearing attached to a bearing holder that is fixed to a boss of the flexible external gear. In the bearing holder, an elastic leaf spring part that is displaceable in the axial direction constitutes a part that supports a holder body part that holds the bearing. A preload is applied by the leaf spring part to the bearing in the axial direction. Thus, there is no need to separately arrange a preload application member inside the flexible external gear, and the axial length of the hollow strain wave gearing unit can be shortened.