Opening/closing device

An opening/closing device comprising:

• a gate that receives a flow of fluid, and is able to fall down toward a downstream direction of the flow;
• a fall prevention unit that supports the gate, thereby preventing the gate from falling down;
• a support release unit that releases the support for the gate by the fall prevention unit;
• a first float that is arranged on an upstream side of the gate, is arranged on one of the left side and the right side of the gate viewed from upstream, and is less in the specific gravity than the fluid;
• a floating prevention unit that prevents the first float from floating;
• a second float that is arranged on the upstream side of the gate, is arranged on the other one of the left side and the right side of the gate viewed from upstream, and is less in the specific gravity than the fluid;
• and
• a floating prevention release unit that releases the prevention of the first float from the floating by the floating prevention unit as the second float floats,
• wherein the support release unit is activated as the first float floats.