This study aims to evaluate the potential reduction in flood damage costs provided by irrigation reservoirs, assuming a worst-case scenario for damage reduction in Japan. We assessed the effects of irrigation reservoirs on flood damage reduction. When the storage rate was 0%, the damage cost reduction rate ranged from 1.1% to 1.7% by the end of the century. In this worst-case future climate scenario, the prefectures with the highest damage reduction rates included 16 prefectures for farmland out of 47 prefectures in Japan, 3 for residential areas, 2 for businesses, and 26 for golf courses. Notably, most prefectures with high damage reduction rates had golf courses. The findings from this chapter are expected to provide scientific insights into the use of irrigation reservoirs for flood control, particularly in light of climate change impacts across Japan, by prefecture and land use type.