PURPOSE: To allow a flexible film to fall down in a recessed form stably by a method in which projected and recessed portions coated with a lubricant are formed between the fixed line of the flexible film and the projected portions are fixed to the river bed movable to the longitudinal direction of axis.

CONSTITUTION: A flexible film 2 is fixed to the river bed 1 is fixing lines A and B, projected portions 5 are formed by bars 4 on the surface 3 of the river bed, recessed portions 6 are formed between the bars 4. When air is discharged from the flexible film 2 for fall it down, the film 2 is fallen down to a stage as shown by chain lines by water pressure. Since a lubricant such as manganese disulfide, etc., is coated on the projected and recessed portions 5 and 6, the film 2 can be stably fallen down toward the direction of arrow. Also, since the projected portions 5 are coupled at right angle to the longitudinal direction of axis in a movable manner, the film 2 can be easily dropped into the recessed portions 6.