The bearing capacity for a composite breakwater's mound is considered to be a problem of foundation stability in the present study. Expected displacement due to mound failure under eccentric inclined load by repeating wave forces is estimated. The displacement is calculated by use of the movement equation of sliding mass along a failure arc, similarly to Newmark's method in the earthquake engineering. A computation program, which taking into account various conditions for individual breakwater, has been developed, and several analyses were conducted with this program. It has been found that the relationship between expected displacement and designed safety factor of bearing capacity significantly varies with the ratio of width to height for the caisson box. If the deformation allowed design method is introduced, the caisson width will be effectively reduced cspccially in the cases of larger width-height ratios. The program was used in a case study on a failure history recorded in Tomakomai west port. High wave storms invaded when the breakwater was under construction, which was imperfectly protcctcd by uncompleted wave-breaker blocks. It was observed that the caissons near the end of breakwater face line settled 7cm ～ 45cm. By using the high wave record, and also by accounting for increase of wave force due to the incompletely armored wave-dissipating blocks, the back analysis resulted expected settlement and horizontal displacement which agrees with the measurements fairly well.