An opening-closing device (1) provided with a gate (10) which can receive the flow of sewage water (W) while in a standing position and fall over in the downstream direction of the flow, and also with a first spring (52a) which generates force for moving the gate (10) to the standing position. When the gate (10) is in a fallen-over position, the first spring (52a) generates force which is not sufficient to move the gate (10) to the standing position, and when the gate (10) is in a tilted position in which the gate (10) is tilted to an angle equal to or less than a predetermined angle, the first spring (52a) generates force which is sufficient to move the gate to the standing position.
