The floating structures that may be used for such purposes as an international airport or an offshore city are expected to be as large as several kilometers long and wide. For the estimation of hydrodynamic forces due to waves or body motions that will act on such huge structures, a direct application of conventional numerical methods is practically prohibitive, because the required computational burden is enormous. In order to get rid of this difficulty, an approximate numerical method is proposed in which computational time is drastically reduced without appreciable loss of accuracy. Although a direct application of conventional methods is difficult for the reason that the corresponding structure is so large, the method proposed in this paper exploits the very fact that a structure is huge to simplify the calculation. The effectiveness of the new method is demonstrated in comparison with results obtained without any approximations.