A large number of breakwaters were damaged by the Great East Japan Earthquake on March 11, 2011. Many were destroyed by tsunami overflows. In this research, physical experiments were conducted using various breakwater cross-sections to clarify the exact failure mechanisms caused by tsunami overflows. Using the experimental results, the cause for the breakwaters' collapse can be verified using the safety factor of sliding, overturning and bearing capacity of each breakwater. If the overflows scoured the rubble mound, the safety margin was estimated using a post-scouring cross-section. Scouring by overflows decreases the rubble mound's bearing capacity, increasing the likelihood that breakwater would collapse. When the scouring reaches the bottom of the caisson,