PURPOSE: To prevent the blocking of the opening and closing device for water channel with soil and sand by a method in which a flexible plate-like bag is air-tightly connected between the water channels and a means to closely contact the facing inner surfaces of the bag in a separable manner is provided to the outside of the bag.

CONSTITUTION: Between the flange 3a of a discharge pipe 3 in the water channel of a dam and the flange 4a of a discharge pipe 4 is connected with a tank 5, and a connecting pipe 10 and a connecting pipe 11 are connected with a plate- like rubber bag 6 made of a rubber interleaved with two reinforcing layers having a high tensile strenght (e.g., of nylon fiber, etc.). When water is sent into a water pressure pipe 7, the external pressure of the bag 6 is raised. When the water level of the pipe 7 is raised by (h) from the water level of dam, the facing inner surfaces of the bag 6 are closely contacted with each other, and the communication between the discharge pipe 3 and the discharge pipe 4 is interrupted.