Residential buildings are one of the important parts of the urban system. They are dense in the mega-cities such as Tokyo. This paper has analyzed the residential buildings and floor area distribution for Tokyo 23 wards. Methodology was formulated, based on the DOE-2. I E building energy simulation model, to estimate the heat discharges from residential buildings to the outside environment for a representative detached house and an apartment building by accounting for the heat storage within the building structures.