DER-CAM Developed by the Lawrence Berkeley National Laboratory (LBNL), is an optimization tool for DER technology selection. However it can not be simply applied to the Japanese case because of the different climate and the utility tariff. This research aims to develop an optimization tool for distributed energy for Japanese buildings using GAMS, a high-level modeling system for mathematical programming and optimization. This paper describes how we apply and demonstrate the tool to the energy center at Kitakyushu Research city, where has installed a fuel cell and a gas engine. An analysis has also been conducted to see how the utility tarriff and the equipment efficiency can affect the operation of the DER system.