To investigate relation between synchronization of nonverbal behavior and degree of understanding, we examined a task-oriented communication. The task consisted of an explanation from a lecturer to a student. As a nonverbal behavior, head movement was measured using an accelerometer. Degree of understanding was subjectively evaluated by the student for each section of the explanation. As a result, high degree of understanding tended to be observed in the period of negative correlation between each head movement. The result suggested that synchronization of each head movement related with degree of understanding. The result also indicated that degree of understanding was high when speaker's head and listener's head moved alternately.