The promotion of paddy rice planted area is one of the important factors for estimating pesticide concentration in river water. The aim of this paper is estimating a ratio of paddy rice planted area derived from the National Land Numerical Information (NLNI) and statistics of agriculture using FOSS4G (Free and Open Source Software for Geospatial). First, we developed a spatial database about land use and river basin. And we calculated factors to convert paddy categories in NLNI to paddy rice planted area based on agricultural statistics. Then, the ratio of paddy rice planted area of prefectures, river systems and upstream basin area on river discharge monitoring sites were estimated. It is concluded that the estimation based on NLNI and statistics of agriculture reflected characteristic of the upstream basin at river discharge monitoring sites.
