Opening and closing device

This opening and closing device comprises a gate, a fall prevention unit, a support release unit, a first float, a surfacing prevention unit, a second float, and a surfacing prevention cancellation unit. The gate is subjected to the flow of a fluid and can fall in the downstream direction of the flow. The fall prevention unit prevents the gate from falling by supporting the gate. The support release unit releases the support for the gate by the fall prevention unit. The first float is disposed upstream from the gate on one of the left side and the right side of the gate as viewed from upstream and has a specific gravity less than the fluid. The surfacing prevention unit prevents the first float from floating to the surface. The second float is disposed upstream from the gate on the other of the left side and the right side of the gate as viewed from upstream and has a specific gravity less than the fluid. The surfacing prevention cancellation unit cancels, due to the second float floating to the surface, the prevention by the surfacing prevention unit of the first float from floating to the surface. In addition, the support release unit is actuated due to the first float floating to the surface.