CADMAS-SURF/3D is a three-dimensional numerical wave tank under development which has recently been applied to some studies of wave-structure interaction. However some problems have been pointed out: for example it is difficult to determine the reasonable drag coefficient and grid dependency of resistance is remarkable. In this study we clarified grid dependency of resistance using a conventional resistance calculation method and examined the influence of drag coefficient on wave force. Next the applicability of the CADMAS-SURF/3D with a Dupuit-Forhhimer method was investigated through the comparison between experimental and numerical results of wave pressures acting on the superstructure of sloping breakwater.