It is one of the key themata to investigate the circulatory, muscular, nervous and nutritional systems when humans stay for long time in the space environment in order to improve the safety and efficiency in manned space flight. It is well known that immobility and microgravity [correction of microgravidity] induce selective skeletal muscle atrophy. Previous experiments indicated red soleus muscle was selectively involved and the most striking finding was myofibrillar degeneration without necrosis or phagocytosis. This study is focused on the mitochondrial function in this degenerative process because red soleus muscle contains much mitochondria than white muscles, and mitochondrial changes may be closely associated with the core structure observed as the myofibrillar disorganization.