A fluid coupling includes a valve unit for reducing working fluid introduced into a labyrinth-groove to adjust a ratio of a rotation speed of a cooling fan to that of an internal combustion engine, when the engine speed is rapidly increased. The valve unit comprises a through opening formed in one surface of a partition member which divides an internal space within a housing into two fluid chambers, and a valve plate pivotably supported on the surface of the partition member. The pivotal axis of the valve member is on a plane essentially same as that of the surface of the partition member. When no load is applied to the valve member, the valve member is positioned such that the through opening remains open, and, when force applied thereto by rotation of the housing becomes greater than a predetermined value, the valve member is moved so as to close the through opening.