ABSTRACT A multi-level model for the calculation of tidal flow and material diffusion of a bay has been developed to simulate ocean environmental problems Recently, it has been extended to the problem of a bay with a very large floating structure or a mega-float. However, the validation of the application is required before the model can be generalized, because the multi-level model is based on a quasi-three dimensional method with hydrostatic pressure approximation. In this paper, results of a model experiment are compared with those of numerical calculations and the applicability of multi-level model is discussed. In the model experiments, the flow around a towed pontoon was measured. The draft was changed to investigate the effect of draft/water depth ratio, and the mass-flux below a pontoon was estimated from the measured velocity profile. Then measured distributions of velocity and mass-flux below the pontoon are compared with calculations by a multi-level model in which the existence of a mega-float is taken into account. INTRODUCTION There are ideas to use a very large floating structure or a mega-float for utilization of coastal area such as off-shore city, floating airport etc. To realize these ideas, a verification experiment of a very large floating structure or a mega-float settled in coastal area or a bay has being done by setting Mega-float model (Length 300m, Width 60m and Draft 0.5m) in Oppama (Yokosuka, Japan) since 1995. There the researchers of T.R.A.M.(Technical Research Association of Mega-float) have been observing ocean environment changes caused by settlement of Mega-float model. However, some new problems arise because the assumed floating-body size is so huge and can be up to a few kilometers. Kyozuka et al (1995) stated that one of the problems concern the environmental impacts on the surrounding sea area due to setting a mega-float in coastal area or a bay.