Tourism is an essential industry in Okinawa prefecture. The number of tourist increase year by year, with approximately 5.7 million tourists in 2007. Development of the tourism industry and an increase in tourists causes various social problems in this small island society. One major problem is a lack of water. Inhabitants of Okinawa prefecture are troubled with not enough water, because Okinawa has a subtropical oceanic climate and is composed of very small islands in the Ryukyu archipelagos. Some water resource developments have been done since Okinawa's reversion to Japan in 1972. However, water demand has increased with an increase in population and tourists so the risk of drought has increased. This paper focuses on the risk of drought by describing the changes in the regional social environment, water use and availability of water. Particularly, a risk assessment was carried out on the relationships of the tourism industry and quantity of water use. At first the water service enterprise of the island regions and information on water resources were gathered by survey of persons in charge of water supply services, and the relationship of these factors to island social problems determined by a structural model. Second, changes in the social environment due to tourism and effects of the promotion of tourism on water demand were clarified. Third, we analyzed the quantity of water necessary to meet the water demands of tourism. In addition, to it is described influence to give to water use in tourism supplies it with water in limited water supply. This study is a start for social planning of sustainable resort islands, and important for clarifying the carrying capacity of tourism based on water resource management. JEL Classification: Q01, Q25