Past and future trends of Japanese energy demand/supply are investigated in industry, residential and commercial and transportation sectors. Recent growth of the energy demand is getting stable by rapid change of industrial structure from materials and heavy industries to service industries. However, the Japanese long-term energy outlook reports one percent per year of energy demand until 2010 because of moderate energy growth of households, services and transportation. It is inevitable to reinforce energy saving and expanded utilization of alternative energy sources such as natural gas, nuclear power and renewable energy in order to achieve the CO2 emission of the government target up to 2010. The paper also describes expectable measures to reduce energy consumption in each sector, and to clarify life cycle CO2 emissions of electric power generating technologies which could mitigate CO2 emissions.