The authors aimed to develop a practical system to recognize group emotion without constraining users. This paper introduces a method to recognize the emotion of an audience attending a presentation by the ambient sensing of body sways or synchrony of multiple users. We developed ambient sensing chair, which employs wireless sensors to determine the center of force of seated users and a methodology for recognizing individual and group-acting-out behaviors. We designed an experiment to examine the relationship between audience body sways and emotions in a natural situation. Subjects watched five technical presentations, each of which consisted of 20 slides that switched automatically every 20 [s] and responded to questionnaires about their subjective impressions of each slide in each presentation. As a result of comparing subjects' acting-out behaviors and their synchrony to subjective impressions, synchrony could be well correlated to boredom. Our system was able to identify boredom at a recognition rate of more than 80%.