The transport of fine sediment particles, such as silt and clay, has a key role in estuarine and coastal environments. In the study, the fine sediment transport processes in the Ariake bay are elucidated through the analysis of field data and a numerical simulation with a model applied to the bay also. The numerical model uses a multi-level hydrodynamic model to reproduce the tidal current field and an advection-diffusion model with bottom boundary conditions for erosion and deposition to estimmate transport rate of fine sediment. The field measurement indicates that the suspended sediment concentration and their transport rate are prominent in the spring tide period especially in the upper bay and their averaged direction tends to the west ward. ne numerical simulation, considering the tidal current as dominant force, shows that the sediment transport characteristic is well reproduced.