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.