Effects of the Global Warming on water resources systems have to be studied both to water demand system and water supply system. Up to this time, many studies were performed only for the supply system. But water demand system is also affected directly or indirectly. The change of water consumption alters a design maximum water consumption and a design intake rate. This matter changes not only the water source facility system but also the reliability of water supply. Thus the influences on water demand system by Global Warming are very serious. In this paper, we investigate a relationship between temperature and water consumption. Through a study of Temperature-Demand curve defined in this paper, we prove that their relationship is quadratic. Analysis of the temperature distribution reveals that there are some regional characteristics with regard to average temperature. Finally we propose a model to estimate water demandunder several scenarios and evaluate the change rate of water demand from present by the Global Warming.