In the present design method of breakwater, the safety factors for sliding and overturning should be larger than 1.2. But such safety factors do not matter the absolute safety. A few breakwater caissons designed in the present design method cause slide or overturn damages every year.
　In this paper, the occurrence probability density distributions of astronomical tidal level at 96 ports in Japan are calculated with four major tidal constituents. The occurrence probability density of tidal elevation around the high water level is much smaller than the density around the mean sea level. It is necessary to calculate tidal level with four major tidal constituents in estimation of sliding and overturning failure probabilities and expected sliding distance of breakwater caisson.
　The failure probabilities and expected sliding distances of present breakwater caissons are computed by the reliability design method considering the occurrence probability desity distributions of the astronomical tidal level. The allowable failure probabilty is assumed, and the caisson width determined by the reliabilty design method is compared with that by the present deterministic design method.