Sporadic inclusion body myositis (sIBM) is an intractable and progressive skeletal muscle disease of unknown cause and without effective treatment. The etiology of sIBM is still unknown, however genetic factors, aging, life styles, environmental factors may be involved. sIBM is generally refractory to current therapy, such as steroid or immunosuppressants. To elucidate the pathomechanism of sIBM is the most important way to make therapeutic approach. In this review we estimated the prevalence of sIBM in Japan and discuss the pathomechanism of sIBM.