PURPOSE:To pfevent the life of a valve from being shortened by frequent ON-OFF operation of the valve by arranging a water sealing resistance so that the water level in a float case starts movement after the water level in a tank moves by a certain depth and softening the vertical movement of a float and reducing the frequency of opening and closing. CONSTITUTION:When the level of water in a tank which is positioned outside the lower casing 3 of a float 13 lowers below the full water level H1 and reaches the water level H3, the level of water in the lower casing 3 also tends to lower. Since control passage 16 is sealed with water, the liquid surface in a float chamber 18a is maintained as it is, and when the outside liquid surface lowers up to H3, the water sealing of the control path 16 is destructed, and the outside air passes through the path 16 and flows into the float chamber 18, and the level of water in the chamber 18 starts lowering. Since the path 18 has a small hole, a pilot valve port 12 is opened slowly, and a valve plate 10 lowers, and valve ports 5 and 6 communicate and liquid is supplied through passages 7 and 9. Then the level of water increases up to H2, and the valve ports 5 and 6 are closed reversely. The valve operates delayed, and flutter is not generated.
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