Opening/closing device

Abstract

An opening/closing device includes a gate that receives a flow of a sewage, and can fall toward a downstream side of the flow, a fall prevention unit that prevents the gate from falling by supporting the gate, a support release unit that releases the support for the gate by the fall prevention unit, a first float that is arranged on the upstream side of the gate, and is smaller in specific gravity than the sewage, a surfacing prevention unit that prevents the first float from surfacing, a second float that is arranged on the upstream side of the gate, above the first float, and is smaller in specific gravity than the fluid, and a surfacing-prevention release unit that releases, resulting from surfacing of the second float, the prevention of the surfacing of the first float by the surfacing prevention unit. Further, the support release unit is activated resulting from surfacing of the first float.