A hollow-type strain wave gearing unit (1) has a sealing member (14) for sealing a gap (15) opening to the inner peripheral surface of a unit hollow portion (5) passing through in an axis (1a) direction. The gap (15) includes a gap section (15b) between an outer peripheral side end race (48b) on the wave generator side and a boss side end face (108), both faces opposing each other in the axis (1a) direction, and the sealing member (14) is assembled therein. The gap section (15b) is formed in a state partially entering the inside of the inner ring (11b) of the second bearing (11) supporting the wave generator (4). It is possible to realize a hollow-type strain wave gearing unit with a sealing mechanism that is suitable for increasing the inner diameter of the unit hollow portion and for reducing the unit axial length.