To promote the effort for energy conservation, it is also important to estimate the indoor environmental quality from the aspect of office worker's productivity. Several experiments in moderately high temperature environment were mentioned in this paper whose results showed that the effects of thermal environment on task performance were contradictory among the task types, while the subjects complained the feeling of mental fatigue more, and more cerebral blood flow were required to maintain same level of task performance at hot condition than that at thermally neutral condition. For evaluating task performance, cost of maintaining the performance, namely fatigue and mental effort, is important in evaluating and predicting productivity. In experiment with long period exposure, indoor air temperature has effects on workers' performance, and the relationship between performance and complaints of fatigue were shown. Also, two examples of field surveys on thermal environment were described. One survey showed the decline in performance in warmer environment, and other showed the decline in performance in cooler environment.