To provide both advantages of both of a HST type and a HMT type transmissions by providing a clutch, which can be freely engaged and disengaged for connection and disconnection, between an external motor and a second shaft connected thereto, and proving a brake, which can be freely engaged and disengaged so as to practically fix a second shaft to a static case for braking.

In the case where a clutch 66, which is provided between an input shaft 14 of a differential hydraulic motor 01 and an external motor driving shaft 40, is disengaged, and a built-in brake 60 in braking condition so as to constrain the input shaft 14, fluid is supplied from an external hydraulic pump 02 to external hydraulic ports 85, 86 so as to have an equivalent function with a HST type transmission. In the case where the clutch 66 is engaged, and the brake 60 is disengaged so that the input shaft 14 is mechanically connected to an external driving shaft 40 for drive, equivalent operation with a HMT type transmission is performed. Function of a HST type transmission and a function of a HMT type transmission can be freely selected in response to the engaging/disengaging condition of the clutch 66 and the brake 60.