To promote the effort for energy conservation, it is also important to estimate the indoor environmentalquality (IEQ) from the aspect of office workers productivity. In this paper, a field survey in a call-centerwas explained with the main result relating the indoor air temperature and the performance of thecommunicators. Then, an example of economical effect of changing the preset temperature of theair-conditioning system in summer was estimated based on the findings from the field survey. It wassuggested that the benefit by the reduction in the running cost and the reduction of the CO2 emissionwould be minimal compared to the possible cost generated by the performance decrement. Thereduction in the emission of greenhouse gases is essential for the world wide concern of the globalwarming issue; however, it is also important to give considerations on productivity for the measures inoffices. Finally, a tool was introduced to measure objectively the performance level of the office workerswith several standard tests.