Various researches highlight that indoor air quality affects performance of workers in offices. Although good indoor air quality improves work performance, it also increases energy consumption. In this study, we achieved a field survey in a real office in order to investigate the relationship between thermal environment, productivity, and energy consumption. In addition to the monitoring of indoor environmental quality and energy consumption, subjective experiments were conducted. In order to evaluate subjective performance, workers and students responded to questionnaires, and to evaluate objective performance, students simulated three types of office works. By calculating room temperature and worker's subjective performance, we demonstrate the correlation between room temperature and worker's performance (R2=0.22, p<0.01). Results show that the room temperature that maximizes worker's performance was 25.7°C.