Device producing a flushing surge

An opening/closing device 1 includes a gate 10 that receives a flow of a sewage W, and can fall toward a downstream side of the flow, a fall prevention unit 20b that prevents the gate 10 from falling by supporting the gate 10, a support release unit 22b that releases the support for the gate 10 by the fall prevention unit 20b, a first float 18 that is arranged on the upstream side of the gate 10, and is smaller in specific gravity than the sewage W, a surfacing prevention unit 44 that prevents the first float 18 from surfacing, a second float 16 that is arranged on the upstream side of the gate 10, is arranged above the first float 18, and is smaller in specific gravity than the fluid, and a surfacing-prevention release unit 42 that releases, resulting from surfacing of the second float 16, the prevention of the surfacing of the first float 18 by the surfacing prevention unit 44. Further, the support release unit 22b is activated resulting from surfacing of the first float 18.