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 three of self-governing bodies. And, forecasting precision is leaded approximately 2%. In this paper, flexibility of the forecasting methods concerning water demands is examined using data of value of water supply per day offered by Saitama City. From this examination, purpose of this study is to lead factors which improve the forecasting precision.