Eosinophilic polymyositis is a rare form of inflammatory muscle disease associated with peripheral blood and/or tissue eosinophilia. Most patients respond to glucocorticoids, however some show poor prognosis, leading to mortality. A 28-year-old female was admitted to our hospital because of myalgia and motor weakness of the upper and lower extremities. Laboratory findings showed significantly elevated levels of muscle enzymes and inflammatory lesions in both thigh muscles were demonstrated on magnetic resonance imaging. A diagnosis of eosinophilic polymyositis was based on histological findings, which showed diffuse eosinophilic infiltration into perivascular spaces in the endomysium and perimysium, and necrosis of myofibers. High-dose methylprednisolone treatment improved the clinical symptoms and muscle enzymes. However, on tapering the glucocorticoid dose, clinical and laboratory findings were exacerbated. After the addition of methotrexate and azathioprine, the symptoms and muscle enzymes recovered without relapse. Here, we report on a case of eosinophilic polymyositis, which responded to immunosuppressants.