Recently, companies have been promoting energy saving in not only their factories but also their offices by controlling office environment such as temperature of air-conditioning system and lighting. On the other hand, the drop in productivity of office workers caused by the energy saving may become a big problem because personnel expense is a large proportion of total cost of the companies. However, since the method to evaluate the productivity of office workers has not been established, it is difficult to estimate the influence on their productivity caused by the energy saving. In this study, therefore, a performance test, CPTOP(Cognitive Performance Test of Productivity), has been developed as a simulated task set of office work in order to evaluate work efficiency of office workers objectively and quantitatively. In addition, a lighting control method has been proposed which adjusts the circadian rhythms of office workers and gets rid of decrease of arousal level in the afternoon by exposing high intensity of lighting in the morning in order to improve their productivity. A basic experiment was conducted to confirm the effectiveness of the proposed methods. In the experiment, six subjects did the performance test, CPTOP, under the lighting conditions of 1,400 lx, 700 lx and 2,100 lx for a week respectively. As the result, it was fount that the task performance of CPTOP under 2,100 lx was significantly higher than that under 700 lx and more than 9% of improvement of task performance was achieved.