In recent years, an international community has been trying to reduce greenhouse gas emissions and fossil energy usages in a variety of endeavors. In particular, a comprehensive and long-term policy for reducing building energy use is required since the building energy takes the biggest burden among the greenhouse gas emissions and the energy use. Advanced countries in the area of the reduction of building energy consumption, Germany, United Kingdom, United States, etc., had already established a long-term strategy and implemented the strategy to policies. EU countries established pan-national dimensions of policies such as EPBD (Energy Performance of Building Directive). A variety of regulations and supports for reducing the building energy consumption are implemented in our nation, and the corporate-driven new construction-oriented policy has been converted into the policy for improving energy performance based on existing public buildings. In this work, we propose a scheme to cut down the energy use for the existing public buildings through simulation studies. SBD (Simulation Based Design) is set up based on analysis of the existing public buildings and energy simulation tasks are performed. Then, a methodology is presented to improve the building energy efficiency implementing energy-saving elements obtained from the energy simulation tasks.