An opening/closing device 1 according to the present invention includes a gate 10 that receives a flow of a sewage W in an upright state, and can fall toward a downstream side of the flow and a first spring 52a that generates a force for bringing the gate 10 into an upright state, wherein the first spring 52 generates a force insufficient for bringing the gate 10 into the upright state if the gate 10 is in a fallen state, and generates a force sufficient for bringing the gate into the upright state if the gate 10 is tilted by an angle equal to or less than a predetermined angle.