We report a case of Mikulicz's disease, IgG4-related plasmacytic syndrome, successfully treated with corticosteroid and mizoribine, followed by mizoribine alone. A 56-year-old woman experienced persistent bilateral swelling in the eyelids and submandibular region accompanied by lymphadenopathy without pain. The patient was diagnosed as having Mikulicz's disease since her bilateral lacrimal glands showed persistent swelling and biopsy specimens of the submandibular glands and lymph node showed diffuse infiltration of lymphocytes and plasma cells. Symptoms and laboratory data were improved with corticosteroids (30 mg). However, after tapering prednisolone to 5 mg, submandibular swelling developed. Adding mizoribine resulted in clinical improvement. Prednisolone was tapered and finally discontinued. Since the levels of IgG4 in peripheral blood were increased even after corticosteroid therapy and IgG4 was positive in a submandibular gland specimen obtained before the start of corticoid therapy. Thus, this case may be associated with IgG4. The present case is the first case in which mizoribine was effective for Mikulicz's disease and IgG4-related plasmacytic syndrome.