This study aims to assess the thermal conditions of an indoor environment deemed unacceptable by workers. For this purpose, Ostracon, a voting device, was developed to record the physical environment at the time a worker presses a button to express a complaint. Ostracon was used to record the opinions of 90 workers about their thermal environment in six offices during the summer. The results show that workers found the indoor thermal environment unacceptable even when the static thermal conditions were within a range that was predicted to be comfortable. Moreover, most of the workers' complaints were expressed moments after returning from tasks performed outside the office. This suggests that the workers' complaints were influenced by factors other than the indoor environment such as thermal history.