The present paper proposes an estimation method for the evaluation of the degree of instability of a caisson type breakwater. The degree of instability is shown by the value of encounter probability of slide failure during the life time of the breakwater. The encounter probability is computed under the assumption that the estimation errors of deep sea wave, wave transformation, the wave forces and friction factor should be expressed by the normal distribution. The water elevation due to storm surge and tide affects the weight of the caisson in water and wave height acting upon the caisson. These occurrence probability are also introduced into the computation. Various computations are carried out in order to investigatethe characteristics of the variations of the encounter probabilities due to water depths at the breakwater, slopes of sea bottom, and tidal levels.