Anti-muscle-specific receptor tyrosine kinase (MuSK) antibody-positive myasthenia gravis (MG) patients show various responses to conventional immunosuppressive treatment and some patients are resistant to these therapies. We report a 50-year-old Japanese man with anti-MuSK antibody-positive MG, who showed no or poor response to various therapies, including plasmapheresis, corticosteroid, and tacrolimus. The patient was then treated with intravenous immunoglobulin (IVIG), and showed a good response that persisted over 20 months. The outcome of this case suggests that IVIG treatment may be an effective therapeutic option for anti-MuSK antibody-positive MG, with a potentially long-term effect.