Recently, energy saving in building sector is of great important to meet national $CO_2$ reduction goals in Korea. In this sense, it is crucial to improve the energy performance for existing building. The energy efficiency investments, ESCO business, can be beneficial to expand the chance of energy retrofit for existing building. However, ESCO business hardly expand in Korea because it has many limitations in guarantee for energy saving. Also, it is more difficult to apply the energy retrofit measures in existing building than new building. Therefore, the retrofitting methods for existing small-and medium-sized buildings should be improved to initiate ESCO business and expand it in Korea. In this study, the energy retrofit methods for small sized office building were suggested and the case study for a small sized office building located in Suwon was accomplished.