Human communication contains not only explicit factors like the meaning of utterance but also implicit factors like body motion. We presume that the synchronization of body motions has a long time with an increased consensus degree during consensus building, and conducted a conversation task to verify this presumption. The present study focuses on the positive correlation of head motions as an indicator of body motion synchronization and questionnaire evaluation as an indicator of consensus building to analyze the relationship between head motion synchronization and consensus degree. Our experimental results showed that two participants' head motions synchronized with each other and the synchronization time of the head motions has a long time during the period of high consensus evaluation. The results suggest that the synchronization period has a long time as consensus degree increases in the process of consensus building.