PURPOSE: To inexpensively arrange the title device and to enhance the purifying function thereof.

CONSTITUTION: In a three-surface water channel A having a recessed cross section consisting of both left and right side walls and a bottom wall, the lower parts of both inner surfaces of the side walls of the water channel are made thick to provide small stepped parts and a lid 2 is stretched over the left and right small stepped parts to be fixed thereto to constitute an upper water channel 3 and a lower water purifying water channel 4 to the three-surface water channel 3. Purification is promoted in the lower water purifying water channel 4 and the amount of flowing water is ensured in the upper water channel 3 above the lid 2 at the time of flooding.