The rapid progress of industrialization and urbanization due to economic growth and concentration of social function in the urban areas have made the urban environment worse and worse. In most of urban cities, it has become more and more distinct that the increase of energy consumption is causing environmental problems including the temperature rise in the urban atmosphere(urban warming), and air pollution. The present paper reports the results of field observation and three-dimensional simulation of urban warming in Tokyo metropolitan area. The three-dimensional governing equations in an urban atmospheric boundary layer were formulated by virtue of vorticity-velocity vector potential method. A particular attention was focused on the representation of a buoyancy term in equation of motion in the vertical direction, thereby describing the crossover and stratification effects near the ground surface. The vorticity-velocity potential method is superior from the view point of numerical stability and suitable for the simulation of an urban heat island. We first made a survey on the energy consumption in Tokyo metropolitan area. Next, the three-dimensional simulations were carried out using these data. The simulation results were then compared with the data of field observation of the surface temperature by automobiles. Further future prediction of urban warming was performed when the energy consumption rate is increased five times as large as the present rate, which will correspond to the year 2030 if the present consumption rate were maintained until then.