When the water quality of a bay is considered, the concept of exchange of sea water is useful. In recent years, there are some projects of constructing very large floating structures such as ocean airports. The exchange ratio of sea water can be an important index in evaluating influences of very large floating structures over marine environment. The main purpose of the present study is to investigate the influences of large-scale ocean structures on exchange of sea water by means of numerical experiments. At first, the results of numerical calculations were compared with field observation and hydraulic model experiments. There have been a lot of studies about the numerical simulation of coastal currents, but many of them examined about only the characteristic current pattern under constant boundary conditions. From the viewpoint of the reliability of the numerical simulations, it is necessary to investigate more whether the model can reproduce real phenomena in a certain period. In the present study, numerical experiments of exchange of sea water in Tokyo Bay were carried out using real changing atmospheric conditions such as wind and atmospheric temperature as external forcing. The tidal exchange ratio obtained by the calculation was compared with that by the field observation. At the same time, the comparison was also made about current and salinity distribution in the scene area. It showed that the exchange ratio of sea water by the numerical calculation agreed well with the observation, and the calculation could also approximately reproduce the observed current and salinity distribution. The numerical model was also applied to the calculations of tidal exchange in a bay with or without a breakwater. It is found that the influences of the breakwater by the numerical model have the same tendency as the results of the hydraulic model experiments. Therefore, it is concluded that the exchange ratio of sea water can be evaluated properly by the numerical model used in the present study. By using the same numerical model, some numerical experiments were carried out in order to investigate the influence of a very large floating structure on tidal exchange of sea water. The results showed that the tidal exchange of sea water is promoted by the circulation caused by the structure, while it is suppressed because of the reduction of section area of the bay when a large-scale ocean structure is installed.