The mangrove sediment was observed by a core（8 sampling points）and trench（3 sampling points）investigation at the Funaura Bay, Iriomote Island, Okinawa Prefecture. The age of a mangrove roots or shells including the core samples were measured by the ^14C method. Fine sand with abundant mangrove roots underlies the soil in the Backshore. The sediment of the mangrove swamp is mainly composed of weakly laminated mud. Tidal flat deposits consist of massive fine to medium sand. The section of the Funaura Bay is drawn based on these observations.