In response to today's environmental and energy issues, the civilian sector desires a reduction of CO_2 emissions as well as energy saving. In order to deal with such issues, power generation systems which have better performance are desired. This paper suggests a combined power generation system using Stirling engine and closed Brayton cycle machine (Brayton engine). Brayton engine is suitable for effective application of existing waste energy such as cogeneration systems and boilers. Performance of the combined system is improved in comparison with the Stirling engine generator. In addition, characteristics of an expander installed in the proto-type system are inspected.