<p>The causes of crew members' fatigue have been still unclear, though their fatigue was reported as the one of the causes of marine accident. In our previous study, we found that passengers had an exercise load attributable to their postural motion caused by their postural control. However, the muscles that caused postural motion were not clarified. This study investigates the behavior of passengers' postural control against ship motion using surface electromyogram (EMG). Behavior of postural control was evaluated by linear and angular accelerations, exercise load, and muscle activity. Muscle activity was calculated by integrating the surface EMG of the soleus, the vastus lateralis, the lumbar paraspinals and the cervical paraspinals. In addition, regularized muscle activities were calculated by dividing muscle activities at sitting or standing posture by minimum value of these activities in each posture. When passengers were standing, muscle activities of the soleus and the vastus lateralis were larger than the activities that they are sitting posture. We thus found possibility that passengers controlled their postures by using lower limbs.</p>