To cause neutral return moderately with providing resistance, when operating force of a movable swash plate is cancelled and prevent entraining cavitation into hydraulic circuits between a pump and a motor in a hydrostatic transmission for draining pressure oil inside the hydraulic circuits.

Oil ducts 5e, 5f which communicate with the hydraulic circuits 5a, 5b, are open to outer faces on the same side of a center section 5. Piston bodies 64U, 64L freely entering/exiting from the outer faces are mounted on the opening and a valve element 71 for opening/closing the oil ducts 5e, 5f is mounted thereon for stretchably mounting a spring 72 for energizing the valve element 71 in the closing direction between the valve element 71 and the piston body 64U (64L). A member 39c in association with the movable swash plate is faced with the piston bodies 64U, 64L. The member 39c guides the pressure oil to an oil reservoir via a restrictor 80, when the movable swash plate is near a neutral position and the pressure oil generated on the high pressure side of the circuits pressingly opens the valve element 71 against the spring 72 and flows inside the piston body 64.