Runoff water during rainfall was collected at almost the same time in six watersheds (four abandoned farmlands, a paddy field and a forest), and the water quality between these points was compared. As a result, the water quality from abandoned farmland was sometimes much higher than that of paddy fields under cultivation, and the average water quality in 11 rainfall events increased as the rate of abandoned farmland increased. In addition, suspended solids from abandoned farmland during rainfall contained a certain amount of bioavailable phosphorus, and in some cases this concentration was much higher than that of phosphate. However, there is still a problem in using the forest watershed as a background for water quality.