The author has analyzed the creek water quality of the Shiroishi plain in two different times; the first analysis took place around 1960 when the creek water was reasonably kept clean with devoted efforts of the farmers, and the second analysis was conducted around 2000 at which point the farming crops and the living standards had been changed after the development and improvement of the farmland in this area. Following these analysis, the author formulated two typical nitrogen circulation models representing around 1960 and 2000 respectively in order to keep tracking changes of nitrogen circulation in the Shiroishi plain using the biomass recycling diagnosis model which had been contrived in an effort to enhance biomass utilization. These two typical nitrogen circulation models are characterized by three unique features: 1) placing creeks at key locations of the typical nitrogen circulation models; 2) irrigating paddies directly via the creeks irrespective of water sources i.e. irrigation ponds and groundwater, thereby irrigated water is returned to the creeks; and 3) a unique gray water treatment system so called "YUDONE", which was commonly accepted in this area around 1960, was taken into account in the nitrogen circulation. A paradigm in which changes of creek water quality has been hardly discussed from the view point of undergoing changes in the nitrogen cycles in this area. The author hopes that the two nitrogen circulation models of the Shiroishi plain discussed heretofore will not only help facilitate quantitative analysis of deteriorating factors of the creek water but also facilitate proactive approach to the improvement of the creek water quality.