Abstract: There is provided an HST in which a hydraulic pump unit operatively driven by a driving power source and a hydraulic motor unit arranged spaced apart from the hydraulic pump unit and operatively driving a driving-wheel are fluidly connected by way of operation fluid lines so as to form a closed circuit; wherein the hydraulic motor unit is provided with motor-side operation fluid passages forming one part of the operation fluid lines; a motor-side charge fluid passage having a first end opened to an outside to form a motor-side charge port and a second end fluidly connected to at lease one of the motor-side operation fluid passages; and a check valve interposed in the motor-side charge fluid passage so as to allow the fluid to flow from the motor-side charge port to the one motor-side operation fluid passage while preventing the reverse flow.