To realizing such a very large floating structure as an artificial island, for example, a floating airport, it is indispensable to examine comprehensively various expected environmental impacts on the sea area surrounding the floating structure. As a national research project in Japan, a large floating structure called Mega-Float model has been completed and moored off Oppama in Tokyo Bay in July 1996. For the purpose described in the beginning, measurement of temperature and salinity of the sea water in the vicinity of Mega-Float model was started in August 1996, and is still now under way, as of July 1997, in cooperation between a group of university researchers and Technological Research Association of Mega-Float. Based on the results of almost 11 month field measurement, difference in the vertical profile of water temperature at the center and at the northwest corner of the structure is discussed in order to manifest the primary effect of the presence of a very large floating structure on oceanophysical environment around the structure.