To quantify material balances in the estuary centered in mangrove forest, field observations in Ishigaki Island were conducted at Fukidou river. As a result, water budget in the river was controlled by tidal exchange at estuary and the input budget from upriver was not dominant for the total budget even if it's rain. From estimation of SS, T-N and T-P budgets, they all were flowed in the river from upriver significantly at rainy day and accumulated in mangrove forest. T-N budget showed different behavior from T-P. In addition, ammonia nitrogen was dissolved from mangrove forest and flowed out to estuary under some conditions.