It is important to effectively utilize the waste heat from the electric generator in order to achieve high overall energy efficiency of cogeneration system (CGS). To examine the energy conservation efficiency of CGS on the actual situation, we have researched the operative situation of the PAFC-CGS that has been installed in the new building in the university. As a result, the primary energy consumption of the building was found to be 52.8% less compared to the benchmarks of DECC. And the annual average of the thermal efficiency of the PAFC-CGS was found to be 0.54.