Field observation was performed at a neap tide to know characteristics of flow and transport of nutrients in a mangrove area which locates in Ishigaki Island, in which geographical features, vegetation, water qualities, characteristics of soils and flow were measured by using various devices. It was found that the concentrations of nutrients (PO4-P, DOC) in the groundwater of the swamps were 3-8 times larger than those in the main channel. The nutrient flux caused by groundwater flow was estimated by field measurements and numerical simulations. As a result, the amount of nutrients transported from the swamps to the main channel by the groundwater flow was the same order as the nutrient increases along the river flow, indicating that the flux of nutrient due to the groundwater flow played an important role in supplying nutrients at neap tide.