An opening-closing device (1) comprising: a gate (10) which can receive the flow of sewage water (W) and fall over in the downstream direction of the flow; a fall-over prevention section (20b) which support the gate (10) to prevent the gate (10) from falling over; a support release section (22b) which releases the support of the gate (10) provided by the fall-over prevention section (20b); a first float (18) which is disposed on the upstream of the gate (10) and has a lower specific gravity than the sewage water (W); a rising prevention section (44) which prevents the first float (18) from rising; a second float (16) which is disposed upstream of the gate (10) and above the first float (18) and has a lower specific gravity than fluid; and a release section (42) which, when the second float (16) rises, releases the prevention of rising of the first float (18) provided by the rising prevention section (44). When the first float (18) rises, the support release section (22b) activates.
