This paper presents a district energy system simulation model in which the energy flow of a district is modeled as the sum of total energy flow in each building. By using this model, various kinds of energy supply systems can be evaluated taking into account the relationship among performance of energy supply systems, available technologies, and conditions of targeted districts, such as characteristics of energy demand, size and arrangement of buildings. In the last part of this paper, results of a case study, which evaluates two types of energy supply systems utilizing CGS are presented.