There was a few research of investigating sedimentation immediately after flooding in mountain torrent because of the difficulty for its investigation at field site. Particle size distributions of sediment along the stream channel of the Oyabu Creek, Miyazaki, Japan, caused by 1997 flood event was investigated here. Particle size was measured at 30 sites using the line sampling method and mean diameters along the channel were calculated at each point. Mean diameter of sediment along the stream was dominantly averaged at 31.4 ± 9.2 mm and these were resuleted to be transported by tractive force at 8 ~ 14 kg/m^2 during the flood. At the five sites which were located at confluence points, the mean diameter of sediment was averaged at 100.2 ± 28.3 mm and sorting coefficient was averaged at 88.1 ± 8.6 mm. Quick changes in discharge from tributaries (average slope<0.25) and sediment production from tributaries (average slope>0.25) were resulted to influence to the irregulerity in particle size at the confluence points.