This randomized, double-blind, placebo-controlled trial will carry out to assess the efficacy of GB-0998 in the treatment of the steroid-resistant polymyositis and dermatomyositis based on the changes in manual muscle strength (MMT) scores as primary endpoint, and in addition, to assess the safety of GB-0998.

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.