To promote the effort for energy conservation, it is also important to estimate the indoor environmental quality (IEQ) from the aspect of office worker's productivity. In this paper, a field survey in a call-center was explained with the main result relating the indoor air temperature and the performance of the communicators. Then, an example of economical effect of changing the preset temperature of the air-conditioning system in summer was estimated based on the findings from the field survey. It was suggested that the benefit by the reduction in the running cost and the reduction of the CO 2 emission would be minimal compared to the possible cost generated by the performance decrement. The reduction in the emission of greenhouse gases is essential for the world wide concern of the global warming issue; however, it is also important to give considerations on productivity for the measures in offices. Finally, a tool was introduced to measure objectively the performance level of the office workers with several standard tests.