A 48-year-old woman who had bilateral swelling in the eyelids and submandibular region was admitted. Clinical findings suggested that her renal function had deteriorated. Laboratory data showed renal insufficiency (2.52 mg/dl), hypergammaglobulinemia (IgG 3,729 mg/dl, IgA 124 mg/dl, IgM 73 mg/dl). Gallium-67 scintigram indicated abnormal uptake in bilateral lacrimal glands, submandibular glands, and kidneys. A diagnosis of Mikulicz's disease and interstitial nephritis was made, since biopsy specimens of her lacrimal gland and minor salivary gland showed diffuse infiltration of lymphocytes. In addition, renal biopsy specimens showed diffuse severe interstitial infiltration of IgG4-positive mononuclear cells. Symptoms and laboratory data normalized in response to methylprednisolone semi-pulse therapy and prednisolone 50 mg/day. Mikulicz's disease was recently reported to be IgG4 associated disease. In our case, Mikulicz's disease complicated with diffuse severe interstitial nephritis was successfully treated by corticosteroid. The present case supports the hypothesis that IgG4-related autoimmune disease could be causes of Mikulicz's disease and interstitial nephritis.