The Shiroishi plain lying west of Saga prefecture in Kyushu, Japan, is a plane paddy area on a reclaimed land with man-made creeks in various directions over the land. Of recent, it became apparent that water quality in the creeks has been deteriorated ever since the 60's when creek water was reasonably clean. Around 1960 and 2000, the author investigated the nitrogen concentration in the creeks based on the nitrogen circulation model created by the author. The investigation proved that contamination of the creek water was mainly caused by increased quantity of gray water and attendant changes in treatment processes, modification on the operation and maintenance system of the creeks, and increased nitrogen concentration in the creeks attributable to changes in cropping patterns. To be more specific, the area of paddy field has been decreasing due to not only reformation of ill-drained paddy field into well-drained paddy field but also the conversion of paddy field to upland field. Therefore, the expanding upland cropping, which requires more fertilizer, is obviously one of the major factors that increases nitrogen concentration in the creek water. Based on these investigations, the author analyzed the effects on purification of creek water by way of taking irrigation water from the river Kase, removal of sediment from the creeks, and enforcement of gray water treatment as well as reduction of (chemical) fertilizer in the farmland. One of major findings in the analysis was that water purification of the creeks depends largely on to what extent the use of (chemical) fertilizers can be reduced. Meanwhile it appears that taking irrigation water from the river Kase is a promising measure for improvement of the creek water quality.