Forecasting the water demands is very important for pure-water reservoirs. Because the value of water supply to the water supply stations cannot be changed frequently according to the variation of water demands. The goal of this study is to propose forecasting method of water demands applying all over the country in Japan. In previous studies, we proposed four kinds of methods of forecasting the water demands using data of value of water supply per day offered by two of selfgoverning bodies. And, forecasting precision is leaded approximately 2%. In this paper, to improve forecasting precision, development of forecasting method concerning water demands is examined using data of value of water supply per day of Okinawa prefectural. In Okinawa prefectural, there are some unique matters as compared with other prefectures in Japan. Therefore, it considers that if forecasting precision of this examination becomes higher than that of previous studies, forecasting method concerning water demands is developed. As the results of this examination, it is found out that forecasting precision of this examination is equivalent to the forecasting precision of previous studies.