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Recently, evaluation of an agricultural basin for mass balance has become important. Various materials, soil especially, flow out from agricultural basins and cause water pollution in coral reef seas in Okinawa. In this study, the effects in a sedimentation tank have been investigated to evaluate soil runoff from an agricultural basin. This paper reports on the study. For the purpose of this study, field research was carried out using a real sedimentation tank for three rainfall events. Precipitation, discharge, and water sampling during a rainfall event were observed, and suspended soil concentration (SSC) and soil grading of the samples were analyzed. As a result, Output SSC was lower than Input SSC because of settling of soil particles and dilution of input turbid water. It was also found that most of the input coarse soil particles were deposited, and most of the soil particles flowing out from the sedimentation tank were under 75 µm in diameter. Trap efficiency was 51%, 79% and 67% for each event. Moreover, trap efficiency for fine soil was unexpectedly high, and fine soil was the main component of trapped soil. To consider the magnitude of flooding, î, indicating the rate of water replacement, was introduced. The correlation between trap efficiency and î agreed well with previous findings. We conclude that the effects in a sedimentation tank can be estimated using trap efficiency as related to î, and the deposition of fine soil contributes to increasing entire trap efficiency. Trapping of fine soil requires further study.