The purpose of the study is to investigate the efficacy and safety for steroid non-resistant MG patients in a double blind, placebo controlled study.

Inclusion Criteria:

• Clinically diagnosed as myasthenia gravis
• Those whose MG symptoms are well-controlled by the treatment with prednisone
• Steroid non-refractory Myasthenia Gravis: ≧20mg and ≦40mg / alternate day of steroid dose required to maintain

Exclusion Criteria:

• Those who have thymoma or the history of thymoma (Masaoka stage III or IV)
• Patients who received steroid pulse therapy, plasma exchange therapy, globulin therapy or radiation therapy within 12 weeks prior to the initiation of test drug
• Patients who started the immunosuppressant therapy or increased the dose of immunosuppressant within 12 weeks prior to the initiation of test drug.
• Patients who had undergone thymectomy within 24 weeks prior to the initiation of test drug.
• Pancreatitis or diabetes
• Serum creatinine≦1.5mg/dL