[en] The discharge of nitrate nitrogen from a forested catchment was analyzed using a distributed rainfall-runoff model. Observation in rainfall events was carried out in November, 2006 in the upper region of the Ijira River in Gifu Prefecture which is one of the acid rain monitoring points in Japan and East Asia. The distributed rainfall-runoff model carried out runoff simulation of ground water and rain water at the catchment. The model was verified using observed flow rate, and isotope hydrograph separation by the hydrogen isotopic ratio of rain water and river water. The nitrate ion concentration in river water was shown as a function of the residence time of discharge water in the catchment calculated by the model. As a result, in this catchment, it was estimated that the nitrate ion in the river water, which increased at the rain event, was transported by the lateral flow of the rain water at the subsurface and the up-flow of ground water. Moreover, it was estimated that the residence time of the soil moisture, which interlocked with the conditions of rainfall, influenced the discharge of nitrate nitrogen. In this study, it was suggested that dynamic behavior of water in catchment was important when considering the balance of nitrate nitrogen in catchment. (author)