A hydrostatic transmission including a closed fluid circuit is contained together with a deceleration gear mechanism in a housing filled with fluid so as to serve as a fluid sump. The hydrostatic transmission includes a check valve assembly in connection with the closed fluid circuit. The check valve assembly includes a vertically movable valve member, a valve seat, and a draining operation member being movable perpendicularly to the movement of the valve member. The check valve assembly has three functions. A first function of the check valve assembly is to lower the valve member by the gravity and hydraulic pressure in the closed fluid circuit so as to fit the valve member on the valve seat, thereby sealing the closed fluid circuit. A second function of the check valve assembly is to raise the valve member apart from the valve seat because the hydraulic pressure in the closed fluid circuit becomes lower than pressure in the fluid sump, thereby supplying fluid from the fluid sump to the closed fluid circuit. A third function of the check valve assembly is to raise the valve member apart from the valve seat by movement of the draining operation member, thereby draining fluid from the closed fluid circuit to the fluid sump.