The NO3- N concentration of the Mikasa and Tsuri Rivers, both flowing in and around Fukuoka city and being the second-class river, was monitored every two weeks during one year: from July 2002 to July 2003 for the Mikasa River and from December 2003 for the Tsuri River. The present monitoring clearly indicated that discharge from the plant for final sewage treatment was a main source of the NO3- N opllution of river water in both rivers. This indication was supported by the measureremrnt of δ15 N. However, the extent of pollution was more severe in the Mikasa River; the average annual concentration of NO3- N was in the revel of 10mg L-1 , the WHO standard for drinking water, with a large temporal variation at the sites down the river from the plant for final sewage treatment, while the correspnding concentration at the sites up the river from the plant was around 1.0mg L-1. In the Tsuri River, on the other hand, the average annual concentration of NO3- N was below 3.0mg L-1 at the sute just down the river from the plant for final sewage treatment. These differences in the extent of NO3- N pollution of river water between the Mikasa and Tsuri Rivers can be ascribed to the differences between the established plants of the both rivers in the capability of sewage treatment.