PURPOSE: To keep the amount of water from the main waterway to branched waterway constant by a method in which a through hole is vertically provided in a water conveyance tube, a cylindrical hollow valve having openings on its upper and lower ends is inserted into the through hole in a vertically movable manner, and a water-passing notch is provided to the valve.

CONSTITUTION: A static water tank 3 is provided through a water conveyance tube 4 and an effluence tube 5 between the main waterway 1 and a branched waterway 2. The water conveyance tube 4 is projected into the static water tank 3, its end is closed 6, and a through hole 7 is vertically provided in the water conveyance tube 4. Furthermore, a cylindrical hollow valve 8 having openings at its upper and lower ends is inserted into the through hole 7 in a vertically movable manner, a water-passing notch 9 directed from the lower end to upward is provided in the wall of the valve 8, and an overflow weir 11 is provided to the opening on the static water tank 3 of the effluence tube 5 in a vertically movable manner. The valve 8 and the overflow weir 11 are supported at a supporting point 16 and connected to a float 12 vertically moving according to the variation in the water level (y) of the tank 3.