Because treated sewage water contains nitrogen compounds such as NH4-N and NO2+3-N, nitrous oxide (N2O) which is one of the greenhouse gases is generated by nitrification and denitrification reactions in rivers. This study aimed to develop a method for estimating the amount of N2O generated from the nitrification of residual N compounds in treated sewage water by subtracting the amount of residual N2O dissolved in the treated sewage water. A year-round field survey of dissolved N2O (D-N2O) in the treated sewage water and river water showed that nitrification proceeded in the downstream direction in the rivers following the discharge of the treated sewage water, thereby generating N2O. The deaeration rate of N2O dissolved in the river water was obtained from a laboratory experiment. The N2O emission factor that can be applied to in-river nitrification of NH4-N contained in the treated sewage water was calculated to be 0.0025 kgN2O-N kgNH4-N-1.