Very large floating structures have been proposed for various applications, e. g. floating airports, cities and plants. Among them are semi-submersible type structures. This paper presents a computational method for analyzing responses of very large semi-submersibles in waves. When designing usual semi-submersibles, we need to model and analyze them as three-dimensional frame structure. The output data from this analysis is used as input at the next stage. This will be also true even in very large semi-submersible case. Because of their enormous size, some techniques are needed for such frame analyses. We have extended the formerly developed method by the authors applying sub-structure method. This is rational because the structure is expected to be composed of repetitions of simular structures. We newly introduce the concept of group body by which several columns are treated as one body in terms of hydrodynamics. The unknowns concerning both hydrodynamic and structural equations can be reduced greatly in number. In this process, both hydro-elasticity and hydrodynamic interactions among floating bodies are considered. The results obtained by the present method are compared with the results of the formerly developed method for validity check. We show the 3, 000 (m) structure case. The results are shown to be applicable to zoom-up FEM analyses of local structure as input data.