A numerical procedure is described for predicting the motions and the structural responses of huge offshore structures supported by a large number of three-dimensional floating bodies of arbitrary shape in waves. The developed numerical approach which is accurate in linear system is based on combination of a three-dimensional source distribution method, wave interaction theory and the finite element method of using the space frame element. Numerical results are compared with the experimental ones of the structural responses of a freely floating semisubmersible type structure supported by 8 lowerhulls and 16 columns in shallow water. The results of comparison confirmed the validity of the proposed approach. The method presented is applicable to huge structures for ocean space utilization which seem to be brought into practical use in the future.