We describe a patient who presented with polymyositis with anti-Jo-1 antibodies at 18 years after the onset of rheumatoid arthritis and was successfully treated with the immunosuppressive drug mizoribine at the time of exacerbation. She had developed diabetes mellitus, cerebral infarction, and myocardial infarction after high-dose steroid therapy was initiated. Therefore, an immunosuppressant was preferred as the second-line agent. Treatment with 150 mg/day of mizoribine and 8 mg/day of prednisolone resulted in eventual normalization of muscle enzyme levels. Mizoribine is a purine antimetabolite that inhibits T cell activation/proliferation and B cell proliferation. The potential efficacy of mizoribine for polymyositis was suggested by this case.