SUMMARYMore and more recent studies of emotion estimation have taken the approach of ambient sensing, where users are monitored without constraining their bodies and without being aware of the sensing system. Few of them, however, focus on audience emotions. We attempted to examine audience emotions, which are difficult to estimate when using only individual data, and which come to be estimated more easily when using ambient sensing of synchrony among multiple users. We developed an ambient sensing chair, which employs wireless sensors to determine the center of force of the seated user, and designed an experiment for an audience watching a soccer game on TV or a technical presentation in order to examine the relationship between audience body sway and emotional states in a natural situation. We found that synchrony could be well correlated with audience excitement and boredom, something not observable in the individual data. These results indicate the possibility of development of service marketing systems that evaluate TV content, and feedback systems that give notice of the emotional state of an audience to a presenter. C⃝ 2015 Wiley Periodicals, Inc. Electron Comm Jpn, 98(4): 11-19, 2015; Published online in Wiley Online Library (wileyonlinelibrary.com).