Though high rates of nitrate (NO 3 À ) leaching from forests are undesirable, the factors significantly regulating stream NO 3 À concentration is not clarified yet. In Japan, not only near metropolitan areas but also the Japan Sea-side area with heavy snowfall is well known for receiving more than 10 kg-N ha À1 year À1 of nitrogen (N) deposition.However, NO 3 À concentration in stream water is relatively low in the Japan Sea-side area compared with its concentration in other areas. We examined important environmental factors regulating stream NO 3 À concentrations at baseflow condition in a large region of Japan, the Kinki region (KIN) including a part of Japan Sea-side (JSK)using Random Forest regression. The amounts of N deposition and precipitation were common regulating factors for stream NO 3 À concentration at baseflow condition. Random forest showed the significant correlation between the factors related to ecosystem N retention and stream NO 3 À concentration at baseflow condition, and it suggests that large N deposited during the growing season was incorporated into the ecosystem in the entire KIN. Heavy rain and snow flush N and wash out N accumulated in the surface soil, causing small N accumulation in forests. Also, large precipitation dilute NO 3 À concentration in baseflows. These things lowered stream NO 3 À concentration at baseflow condition. Especially in JSK, most of N deposed with the heavy snow flushed out during the snowmelt period. We provided the first statistical confirmation using Random Forest regression that N accumulation and cycling in forest ecosystems were related to NO 3 À leaching from forests into streams.