The purpose of this study is to improve inefficient use of rainwater in island area where it uses rainwater to supply the domestic water and to propose rainwater utilizing system that is most appropriate to the characteristics of precipitation in Korea. To accomplish these purposes, Cheju island was chosen as a study area and the design for the roof area and tank size of rainwater utilizing system was based on the result of the relationship between the actual precipitation and domestic water data which was used in the analysis by run theory to use run theory. Since the result of the analysis indicated that the designed rainwater catchment system was operated stably in Cheju island, the same result is expected in other island too. Therefore, if this system is executed together with the roof rehabilitation work of island area, it will bring positive effects on broth the improvement of residental environment and the security of domestic water.