In summer of 1996 and 1997, anomalous sudden fall of sea water temperature and rise of salinity were observed several times, simultaneously off Oppama and off Negishi in Tokyo Bay. These phenomena occurred in a common situation that typhoons stayed for a while in the western part of Japan or passed through it, and consequently, relatively strong blow of southwesterly wind lasted for a few days in Tokyo Bay. On the occasion of the typhoon No. 12 in the middle of August 1996, for example, a salinity increase of about 3 psu and a temperature drop of about 5°C were observed two and a half days after southwester started to blow. These unusual temperature fall and salinity rise were assumed to be attributed to coastal up welling caused by continuous blow of southwesterer in Tokyo Bay. In order to verify this reasoning, use was made of the so-called multilevel model to simulate sea water movement. Results of numerical simulations carried out under the condition of southwester of 10 m/ s indicates that wind-driven up--welling is generated mainly in the western coast of Tokyo Bay, and at the same time temperature fall and salinity rise observed in August 1996 are due to the coastal up - welling. At this point, however, it should be noted that to the contrary the water temperature in the head of Tokyo Bay, for example, off Ichikawa and off Chiba, rises definitely as far as results of numerical simulations are concerned. A huge floating offshore structure (HFOS) is expected to have some effects on oceanophysical environment of its surrounding sea area. Hence, an additional numerical simulation was carried out to reveal such effects under an assumption that an imaginary HFOS of 5 km in length and 1 km in breadth is placed on the sea surface off Oppama. As a result, it is shown that predicted change of sea water temperature and salinity in Tokyo Bay are completely identical to those predicted by taking no account of the imaginary HFOS, although sea water movement in the close vicinity of the HFOS is slightly affected by the presence of the HFOS.