PURPOSE: To culture valuable plants and to clean the river, lake and pond water and waste water by placing the stubs of the valuable plants into a water course having a width, depth and gentle grade and passing the river, lake and pond water or the waste water contg. nutritive salts at a high ratio in this water course.

CONSTITUTION: The water coarse having the width, the gentle grade and 2W20cm water depth is constructed. The river, lake and pond water or the waste water contg. the nutritive salts at the high ratio is passed in this water coarse at about the flow rate at which the stubs of the plants are not flushed away and water-cress, etc., are cultured therein. Then, the river, lake and pond water or the waste water contg. the nutritive salts at the high ratio is utilized to grow the valuable plants such as kushinsai and trefoil, by which the water is cleaned. The deficiency of the nutritive salts is made up by allowing the palnktons and the decomposition product of the excreta of small animals in the water to be effectively utilized by the roots of the plants. As a result, the valuable plants are harvested.