The main purpose of this study was to determine the indoor environment in an office introduced a task/ambient system with ceiling radiation membrane and to reveal the effects of this system on workers' thermal comfort and productivity. For a month in the summer of 2010, a field survey was conducted in an office located in Tokyo. In this survey, indoor environment and operation data were measured, and a subjective survey was conducted about the indoor environment using a questionnaire. As a result, it was found that a personal diffuser that allows a worker to individually adjust the air volume in their area improves their thermal satisfaction and subjective performance. However, more than 60% of the workers were dissatisfied with the thermal environment when the setpoint was at 28°C and they could adjust the air volume (28-Ctl condition). Workers' thermal satisfaction and subjective performance would be improved by taking some measures such as lowering the air temperature setpoint, making the orientation of the personal diffusers adjustable, and increasing the maximum air volume of the personal diffuser.