A 65-year-old male patient who had been diagnosed with diabetes mellitus in 2007 was admitted to Shin-Koga hospital in 2014 due to HbA1c elevation (11.6 %). Since 2011, the patient had complained of general fatigue, dizziness and weight loss and in February 2012 he noticed a mass in his submandibular region. Upon admission, he was found to have proliferative diabetic retinopathy, nephropathy (Stage III) and both peripheral and autonomic neuropathy. An ultrasonographic examination of the patient's submandibular region revealed the enlargement of his submandibular and sublingual glands. In addition, a CT examination revealed diffuse swelling of the pancreas along with the narrowing of the pancreatic duct(s), mediastinal and hilar lymph node swelling, and a retroperitoneal tumor. The patient's serum IgG4 level was as high as 999 mg/dL. A histological examination of a biopsy specimen from his submandibular glands showed dense fibrosis, the loss of acinar cells and plasma cell infiltration. An immunohistochemical examination revealed that the infiltrating cells were predominantly IgG4-positive plasma cells (the IgG4-positive cells accounted for 79.8 % of the IgG-positive cells). Treatment with prednisolone resulted in the significant shrinkage of the tumors, especially those in the submandibular gland. This was accompanied by a significant reduction in the serum IgG4 level. The patient's fasting and postprandial C-peptide levels in meal tests were observed to increase after treatment with insulin and prednisolone.