In order to evaluate present status of endangered Japanese Marsh Warbler, we conducted a breeding bird census along Tone River between 16.0km and 33.0km from the sea on July 5, 2001. The warblers distributed at low densities between 16.0km and 33.0km of the south side, and at high densities between 22.3km and 32.5km of the north side of the river. Three hundred and seventy-five males were counted in this region, but the broad riverbed prevents us to count every bird. We constructed two models to estimate a number of the warblers. Simple estimates were calculated from the coverage of study area in a census (model 1). Stepwise multiple regression procedure chose only the area of reedbeds, Phragmites-Carex complex, and that of Solidago-Phragmites complex as independent variables to explain a number of the warblers inhabiting a 500m section of riverbed (model 2). Estimates of the warbler・s populations are 750 from actual counts, 1,238 from model 1, and 1,196 from model 2 in this region, if sex ratio were unity. The distribution area is expanding and population might become more than a three-fold of the previous census in 1993. The source habitats where the warblers are distributing at high densities should be protected as the preserved area, though two-thirds of the distributing area is still categorized into the unprotected and hunting area.