This study quantifies the contribution through energy consumption, to the heat island phenomena and discussed how  reductions in energy consumption could mitigate impacts on the urban thermal environment. Very detailed maps of  anthropogenic heat in Tokyo were drawn with data from energy statistics and a very detailed digital geographic land use  data set including the number of stories of building at each grid point. Animated computer graphics of the annual and  diurnal variability in Tokyo’s anthropogenic heat were also prepared with the same data sources. These outputs  characterize scenarios of anthropogenic heat emission and can be applied to a numerical simulation model of the local  climate. The anthropogenic heat flux in central Tokyo exceeded 400 W m~2in daytime, and the maximum value was  1590 W m~2in winter. The hot water supply in offices and hotels contributed 51% of this 1590 W m~2. The anthropogenic heat flux from the household sector in the suburbs reached about 30 W m~2at night. Numerical simulations of  urban climate in Tokyo were performed by referring to these maps. A heat island appeared evident in winter because of  weakness of the sea breeze from Tokyo Bay. At 8 p.m., several peaks of high-temperature appeared, around Otemachi,  Shinjuku and Ikebukuro; the areas with the largest anthropogenic heat fluxes. In summer the shortwave radiation was  strong and the influence of anthropogenic heat was relatively small. In winter, on the other hand, the shortwave radiation  was weak and the influence of anthropogenic heat was relatively large. The effects of reducing energy consumption, by  50% for hot water supply and 100% for space cooling, on near surface air temperature would be at most -0.5 degrees Celsius.