This paper describes experimental investigations on mechanical behavior of reinforced concrete (RC) slabs under repeated impact loads, which is often observed in RC walls of concrete caissons for breakwaters. In the experiment, residual structural capacity of RC slabs against punching shear failure was examined by conducting static loading tests after falling-weight impact loading tests. As the results, it was found that punching shear capacity of RC slabs subjected to repeated impact loads decreases by about 20 percents in case of the slabs with no damage immediately before the occurrence of punching shear failure. And, it was found that punching shear capacity rapidly declines after the formation of cracks due to punching shear at the bottom surface of the slabs. In addition, it became clear that number of repetition of impact loading until the punching shear capacity decreases by 20% is correlated to impact energy applied.