After 2011 Tohoku-Oki Earthquake, construction of seawalls with tenacious structure is needed, but there are few studies about change of tsunami force acting on buildings behind the seawalls according to construction of them. So in this study, we looked into the applicability of CADMAS-SURF/3D to calculate tsunami force through reproduction calculation of physical model experiment which was conducted by Arikawa and Oie (2014), and consider about change of tsunami force according to seawalls condition. The results of this study are as follows; 1) The tsunami force acting on buildings behind the seawalls is well reproduced with CADMAS-SURF/3D, 2) Depending on the distance from seawalls, some cases with seawalls have bigger coefficient of pressure α than the cases with no seawalls.