The decay characteristics of waves when they are incident to an array of a large number of cylindrical legs are investigated both experimentally and numerically. The number of legs supporting a very large floating structure which may be used for such purposes as an airport is expected to be so large (around 2000030000) that a direct application of conventional numerical techniques for the estimation of hydrodynamic forces that will act on the structure is practically impossible. However, in some previous works, it has been shown that the computational burden can be reduced drastically if a certain uniformity around a structure is exploited. To be more specific, the flow field around a very large floating structure may be assumed that it is the same as that around a floating structure which extends infinitely in horizontal direction. If a similar assumption can be made in the analysis of hydrodynamic forces on an array composed of a huge number of legs, the analysis may be carried out with a reasonable computational effort. In order to examine this possibility, extensive experiments are conducted. Through the examination of these results and also through the comparisons with theoretical computational results, the characteristics of the wave decay are clarified.