A retaining cylinder is threadedly engaged with a fixed cylinder of a flow rate control valve, by means of a screw section formed on an outer circumferential surface. A fence cover, which is capable of expanding and contracting in the axial direction, is installed between the fixed cylinder and a float rod supported by the retaining cylinder. An upper seal member is provided on the float rod, on the upper side thereof with respect to a valve plug. A lower seal member is provided on the lower side thereof with respect to the valve plug. The upper seal member and the lower seal member have substantially identical diameters. The float rod is displaced under an energizing action of a stepping motor. The flow rate of a fluid is controlled depending on a distance of the valve plug with respect to a valve seat.