This study empirically analyzes the economic validity of agricultural water development in the areas of Icheon City and Yeoju-Gun which are famous for high-quality rice but lack enough agricultural water. The analysis of economic validity is based on the benefit-cost analysis, according to the pre-validity investigation of water resources development suggested by the Korea Development Institute. To put it concretely, this study calculates B/C ratio, NPV, and IRR of the agricultural water development. It adopts a scenario analysis, assuming that the agricultural water development will (or not) change the ratio of rice and other crops. The first scenario assumes that there is no change in the ratio of rice and other crops even after the agricultural water development is preformed. The second scenario assumes that the ratio is close to the national average ratio. The third scenario reflects farmers' opinion to the ratio. The B/C ratios of three scenarios are 0.586, 1.013, and 1.272, respectively, implying that the economic validity of agricultural water development depends on the ratio of rice and other crops. Therefore, it is required to carefully and accurately investigate the ratio of rice and other crops to determine the economic validity of agricultural water development. The contribution of this study is to provide a model for the economic validity of agricultural water development that will be continuously performed by the central or the local governments.