PURPOSE: To raise the durability of a rubber dam by a method in which an air supply and dischage pipe is connected to the upper space of a sectional wall provided in a flexible film dam, the lower space is opened toward the downstream side, and the film dam is raised or fallen down while keeping its stable attitude by sending or discharging air all the time.

CONSTITUTION: A rubber dam 10 is made up of an upstream side skin 11, a downstream side skin 12, and a sectional wall 13 and integrated at a projection 10a. The skin 11 and the sectional wall 13 are fixed with anchor bolts 4, and the skin 12 is fixed over the river bed concrete 1 and the slope 2 by hollow anchor bolts 7 to form an upper chamber A and a lower chamber B. When air is discharged to the chamber A through the pipe 15 from the slope 2, the pressure is lowered, water on the downstream side flows into the chamber B through the bolts 7, and the dam 10 is gradually fallen down in stable condition. As the discharge of air is increased, the water in the chamber B is discharged with speeds and the dam 10 is completely fallen down flatly on the river bed. When air is supplied into the chamber A, the dam 10 is raised while the chamber B is closed.