We report a patient with stiff-person syndrome and insulin-dependent diabetes mellitus with anti-glutamic acid decarboxylase (GAD) antibody, who suddenly complained of diplopia due to dysthyroid ophthalmopathy. Therapeutic efficacy of plasmapheresis and high-dose intravenous immunoglobulin was transient. After starting administration of rituximab, the patient showed obvious improvement of muscle spasms due to stiff-person syndrome and ophthalmoplegia following quick depletion of CD20-positive cells in peripheral blood. The anti-GAD and anti-thyroid antibodies decreased slowly. This drug might be a potent therapeutic option for refractory patients with stiff-person syndrome, particularly in those associated with dysthyroid ophthalmopathy.