Sporadic inclusion body myositis (sIBM), the most common form of myopathy with inflammation in those over the age of 50 in Western countries, is an intractable and progressive skeletal muscle disease of unknown cause and without effective treatment. Muscle biopsy typically reveals endomysial inflammation, invasion of mononuclear cells into non-necrotic fibers and rimmed vacuoles, suggesting inflammation and degeneration are co-existed in the pathogenesis. We estimated the prevalence of sIBM in Japan is 9.83 per million in 2003 and an increase in the number of sIBM in Japan in the decade. In this review, we discuss the diagnostic criteria sIBM in Japan, and describe available treatments and promising new therapeutic strategies. We also provide an update on the current understanding of sIBM pathogenesis.