In order to elucidate the factors controlling the concentration of trace elements (Mn, Fe, Ni, Cu and Zn), water samples were collected at the ebb and flood tides on Sep. 3 and 6 in 2005 from the Urauchi River, Iriomote Island, Okinawa, Japan. The concentrations of Mn, Fe, Ni, Cu and Zn were about the same as those of the water samples from the unpolluted rivers in Japan. At the ebb and flood tides, it was found that the concentrations of the trace elements on the water samples from the lower and middle parts of the river were 1.3-5.2 times greater than those from the upper part. This may be closely related with the groundwater (underflow water and seepage water, etc.), and with the environmental change due to the decomposition of organic matter in the mangrove sediments along with tidal change in water level.