On analysis of wave exciting forces of large scale floating structure consisted of multiple elements, a part of authors proposed a computation method taking hydrodymamic interaction between elements into account, and proved effectiveness of the method by model testes in previous papers.In this method, such an assumption is laid down, which gives a restriction in application of the method, that a fictitious, bottom-mounted vertical cylinder circumscribing one element does not contain any part of the other element itself. Although existence of lower-hull connecting column elements violates the above assumption, it can be proved that the effect of violation is small in case of lower-hull. This fact is confirmed by model tests. The computer program for this method is leveled up for the sake of making large scale computation possible, and the upper limit of computation is discussed.