In breakwaters covered with wave-dissipating blocks, outer walls of the caisson may show local failure in repeated collision of the blocks. Local failure induces drainage of the filling in the caisson so that the self-weight of the caisson would be reduced. This failure of the outer wall decreases stability of the caisson against sliding and harbor calmness. Therefore the design and verification method of the caisson and the preventive measures against such accidental impact loads is necessary to be developed while it has not been established so far. When the preventive measures are implemented, it is important that the life-cycle cost (LCC) would be considered from the viewpoint of strategic maintenance. This paper investigated the advantageous conditions of the preventive measures from the viewpoint of LCC reduction, by comparing the cost for the preventive measures, the cost for post-failure repair and event probability. In addition, the manual for the design method of caissons against accidental impact loads due to repeated collision of the blocks was provided.