The large-scale social experiment, which many citizens including media representative participated for evaluating the effects of the watering on the “Heat Island Effect”, was carried out in Tokyo Metropolitan in Aug. 25 2003. About 14000l. water were sprinkled for about 20 minutes from noon. Air temperature and humidity were measured by moving observation system and fixed observation system at watering area in Eastern Tokyo. After the watering, the air temperature difference between outside and inside of watering area increases, and its difference was about 0.5°C. Temperature of watering area is lower than the temperature of outside area of watering after the watering. The effect of the watering was verified using the numerical simulation based on MM5. As a result of this analysis, air temperature degreased 2-2.5°C in watering area after the watering.