Efficacy and Safety Study of GB-0998 for Treatment of Steroid-resistant Polymyositis and Dermatomyositis (PM/DM)

Inclusion Criteria: * Patients who have been defined as "definite" based on the criteria of Bohan and Peter for polymyositis and dermatomyositis. * Patients who are defined as steroid-resistant ones based on the changes of scores on manual muscle testing (MMT) and serum levels of creatine kinase (CK) during observation period before administration of drug. Exclusion Criteria: * Patients with malignant tumors. * Patients with acute interstitial pneumonia, including acute exacerbation of chronic. * Patients with severe muscular atrophy for a long period. * Patients with severe infectious disease. * Patients who have the anamnesis of shock or hypersensitivity to this drug. * Patients with severe hepatic disorder or severe renal disorder. * Patients who have the anamnesis of cerebral infarction or ischemic heart disease, or who having symptom of these diseases. * Patients who have been diagnosed as IgA deficiency in their past history. * Pregnant, lactating, and probably pregnant patients, and patients who want to become pregnant. * Patients who were administered immunoglobulin dosage within 6 weeks before consent.