Efforts to cope with global warming are concentrating on reduction of energy consumption to be occurred in the stages of resource saving and post construction in the field of architecture. Especially, in case of domestic construction industry, the efforts for decrease of environmental load on existing dwellings should be essential under the situation of that apartment is of the greatest importance. Various policies and systems for energy saving and decrease of environmental load in the country are in progress of complement and development in a wide variety but they are limited only to application for new buildings or utilization of the elementary technology. Also there occur many errors in the aspects of efficiency and economic feasibility regarding the performance and improvement methods for residential environment of green buildings which are newly introducing. In this regard, we are in the situation of that detailed analysis of environmental load on existing buildings and study on the application of design factors for eco-friendly improvement are insufficient. Hence, this study has the purpose of analysis of economic feasibility about energy running cost so to deduce building types being favorable of eco-friendly factors at the stage of post construction targeting apartment. Based on these results, it expects to lead to the realization of green building for existing buildings as it will be able to apply of efficient design factors in improvement of green building in the process of planning and design.