PURPOSE: To reduce the size of a device and to prevent the occurrence of water- hammering during closing of a valve by a method wherein a valve body capable of converting circular movement displacement of a float into slide movement is incorporated in a hollow body having a fluid outlet, and an opening area right before closing is reduced.

CONSTITUTION: A body 1 is provided in an upper part with a fluid inlet 2 and in a lower part with a fluid outlet 3 comprising a number of slit-form openings and having an opening the area of which is small right before closing. A valve body 8 comprising a resilient sheet or a resilient film effecting slide movement through the medium of a float arm 5 resultant from circular movement displacement occasioned by the vertical fluctuation in a level of a float 4 is rotatably contained in the body. A fluorine resin layer is formed on the outer surface of the valve body 8 or the inner surface of the body 1 to cause smooth slide of the valve body 8, and the valve body 8 is lifted up along the outer periphery of a lifting plate 7 fixed to the two ends of a rotary shaft 6. This constitution eliminates a need for the use of a valve body having a large bore and reduces the size of a device.