To realize a very large floating structure (VLFS), it is indispensable to examine various effects on marine environment around a VLFS. For an assessment in the physical, chemical, and biological aspects, a numerical study by means of ecohydrodynamic model, which is an ecological-physical coupled model, is conducted. First, to validate the ecohydrodynamic model, numerical simulations of coastal ecosystem in the summer of Tokyo Bay are carried out under the boundary conditions which change every moment, and under those which are kept unchanged for computations. Additionally, the effects of an imaginary VLFS on the surrounding environment are examined in two cases : the VLFS is assumed to be installed in the sea area off Yokosuka or off Haneda in Tokyo Bay. As a result of some computations, it is concluded that predicted change of water quality agreed quantitatively with observation data, and the impacts of the VLFS on marine environment are localized in close vicinity to the floating structure. However the extent of the effects is relatively larger in the sea area off Haneda, where the residual current is weak, than off Yokosuka.