Recently, several types of numerical wave flume to simulate the non-linear wave transformation have been developed. In this study, the applicability of CADMAS-SURF which is one of them for calculating the wave pressures upon wave absorbing caisson is discussed. The structure is identified by the slit wall and wave chamber, and it is important to set the values of inertia coefficient, resistance coefficient and permeability coefficient when modeling the slit wall as porous body. Then, the sensitive analysis on those parameters, and the calculation of the wave pressures upon the structure with CADMAS-SURF was conducted. And it is shown that the calculation results have a good agreement with the experimental results in equivalent sliding wave force acting against the wave absorbing caisson.